Question

Let's use 2x + y = 5.
Subtract 2x from both sides, giving
y = -2x + 5.
Now use -2x + 5 instead of y in the other equation:
3x – 2y = 25 becomes 3x – 2(-2x + 5) = 25.
Multiply the -2 by the terms in parentheses, giving
3x + 4x – 10 = 25.
Simplify the left side:
7x – 10 = 25.
Add 10 to both sides, giving
7x = 35.
Divide both sides by 7, giving
x = 5.

Substitute 5 for x in one of the original equations (it doesn't matter which one):
3x – 2y = 25 becomes 3(5) – 2y = 25, or
15 – 2y = 25, or
15 = 2y + 25, or
2y = -10, or
y = -5.