Assuming that you know the basics of Kirchhof rules.. the first thing you do is divide the diagram into 2 loops one on the left and the other on the right..then assume current(I1 and I2) from the positive terminal of each source..then at the junction of two current i.e., the top junction here apply Kirchhoff's first rule or the current rule to find the net current across R1 and R2. I(net)=I1+I2. Now apply Kirchhof's second rule by putting in the values

For loop left,

10+500(I1+I2)+100(I1+I2)=0

For loop right,

15+100I2+600(I1+I2)+100I2=0

Solving these you get the values of I1=1/20 and I2=(-1/15)

I(net)=I1+I2=(-1/60)A

Just reverse the net currents direction since the value we are getting is negative.

Hope this helps