From eqn 1, y=5x-12. Substitute this value for y into eqn 2: 3x-2(5x-12)=3. Open the brackets and be careful of the signs: 3x-10x+24=3. -7x+24=3. Add 7x to each side and then subtract 3 from each side: 24=3+7x and 21=7x, so dividing both sides by 7: 3=x, or x=3, same thing. Now find y=5*3-12=3. Substitute these values on the original eqns: 15-3=12 and 9-6=3. Right!