Since the first equation already has an isolated variable, substitute
(-6x + 21) for y in the second equation.
-3x - 3(-6x + 21) = -18
-3x + 18x - 63 = -18
15x = -18 + 63 = 45
x = 3
now plug this value back into the second equation as given
-3(3) - 3y = -18
-9 - 3y = -18
-3y = -18 + 9 = -9
y = 3
So, the ordered pair (3,3) is the solution to the system of equations.
but, remember to check your answer against the first equation
(3) = -6(3) +21 = -18 + 21 = 3
it is generally very quick work to double check your answers.
