Bobby2017: Well done for having a go. You know how to solve a problem using simultaneous equations. Alas, the simultaneous equations you wrote down are incorrect. You made errors because you did not define the currents i1 and i2 and their directions nor the voltage drops across the resistors and their polarity. This meant that when you added up the voltage drops in a loop to equal the electromotive force in the loop you had some signs wrong in the simultaneous equations.
An important lesson then is to define your currents and voltage drops on a diagram as I did top left in the attachment.
Normally in a dc circuit one would use conventional current flow ie from positive to negative. So current 'leaves' the positive terminal of the battery. The polarity of the voltages across a resistor when a current flows through them as I have defined them is as shown by the arrows in my attached diagram where the arrow tip is +ve with respect to its tail - note well that for the currents I have defined the voltage drop polarity (arrow direction) is in the opposite direction.
Applying Kirchoff's voltage law = Sum of emfs in a loop equals sum of voltage drops - for the left hand loop - it would have been helpful if I had remembered to put arrows beside each battery to indicate the direction of the emfs of the batteries. For the 12V battery it would point upwards and for the 5V battery it would point downwards:
12 = 1000i1 + 680(i1-i2)
For the right hand loop:
5 = 1500i2
- 680(i1-i2).......Note well the minus sign before the 680 Ohm resistor because the polarity of the voltage drop I defined across this resistor is in the opposite direction to the voltage drop across the 1500 Ohm resistor.
After deriving the simultaneous equations I then use the matrix algebra method to do the mathematics to arrive at i1 = 9.2375mA and i2 = 5.175mA. Because these are positive values these currents are indeed in the directions I defined in the diagram. A negative sign would reverse the direction of current flow.
One then finds the voltage drop across the 680 Ohm using using Kirchoff's current law. The current through the 680 Ohm resistor is i1-i2, so one can then use Ohm's Law to calculate the voltage drop. And since i1 is greater than i2, (i1-i2) is a positive number which means that the voltage drop polarity across the 680 is indeed in the direction I initially defined. If (i1-i2) was negative this would indicate the voltage drop polarity is opposite to what I defined.
As clear as mud? I hope not.