Solve system by substitution Help

x - 2y = 6
y = 2x - 21

Given x - 2y = 6 and y = 2x - 21
x - 2(2x - 21) = 6
x - 4x + 42 = 6
-3x + 42 = 6
-3x = 6 - 42
-3x = -36
x = -36/-3
x = 12

y = 2x + 21 → y = 2(12) - 21 → y = 24 - 21 → y = 3

21=2x-y
6=x-2y multiply by 2

12=2x-4y
21=2x-y subtract
-9=-3y
y=3 x=12