-3x + 6y = 15 -----------------------(1)
y = 2x + 4 -----------------(2)
substitute equation 2 in (1) and then solve for x
-3x + 6(2x + 4) = 15
-3x + 12x + 24 = 15
9x + 24 = 15
subtract 24 from both-side
9x = 15 - 24
9x = -9
Divide both-side of the equation by 9
9x/9 = -9/9
x = -1
substitute x = -1 into equation (2)
y = 2(-1) + 4
y = -2 + 4
y = 2