To find the value of x, substitute/plug in 6 into "y" since y = 6, then isolate/get the variable "x" by itself in the equation:
2x + 3y = 36 Plug in 6 into "y"
2x + 3(6) = 36
2x + 18 = 36 Subtract 18 on both sides
2x = 18 Divide 2 on both sides to get "x" by itself
x = 9
PROOF
2x + 3y = 36 Plug in 6 into "y" and 9 into "x"
2(9) + 3(6) = 36
18 + 18 = 36
36 = 36