We have the equations:
x + 3 = 15 and 3y - 1 = 2x - 4
x + 3 = 15 |-3
x = 12
Put the value of x to the second equation:
3y - 1 = 2(12) - 4
3y - 1 = 24 - 4
3y - 1 = 20 |+1
3y = 21 |:3
y = 7
