WAEC 2023 · Paper 1 · Q39

If $2x - 3y = -11$ and $3x + 2y = 3$ , evaluate $(y - x)^2$ .

Worked solution (try it first)

Make the $y$ terms opposite: multiply the first by 2 and the second by 3, giving $4x - 6y = -22$ and $9x + 6y = 9$ .
Add them: $13x = -13$ , so $x = -1$ .
Put $x = -1$ into $3x + 2y = 3$ : $-3 + 2y = 3$ , so $y = 3$ .
So $y - x = 3 - (-1) = 4$ , and $(y - x)^2 = 16$ , option C.