Answer:
-12
Explanation:
We are given information to solve for 2 variables x and y. To solve this I substituted 3x for y in the equation 5x - 4y = 21
So substituting gets us 5x - 4(3x) = 21
and we can solve the equation for x
5x - 12x = 21
-7x = 21
x = -3
Now that we have found x, we can find y simply by putting it in the equation y = 3x
y = 3(-3) = -9
Once we have both variables, we can find the answer to x + y
-3 + -9 = -3 - 9 = -12
So the answer to the question
x + y = -12
