Equation 1:
2x + y = 25
-Subtract 'y' to both sides (quick note: pretend 'y' is '1')
2x + 1 - 1 = 25 - 1
2x = 24
-Divide '2' to both sides
2x/2 = 24/2
x = 9
Equation 2: 3y - x = 20
-Add 'y' (or 1) to both sides
3y - 1 + 1 = 20 + 1
3y = 21
-Divide '3' to both sides
3y/3 = 21/3
y = 7
