Y = 3x 5x - 4y = 21
The system of equations above has solution (x, y).
What is the value of x + y?

-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