{y=2x+14
{−4x−y=4

what can i substitute for "y" in the equations?

Since y = 2x + 14 you can substitute (2x + 14) for y in the second equation
-4x - (2x + 14) = 4.

This is an equation with only one variable so you can solve it for x.  Then use that answer to find the value for y.

Distribute the subtraction sign throughout the terms in the ()s.
-4x - 2x - 14 = 4
-6x - 14 = 4            This is how I got -6x:  -4x - 2x = -6x (combine like terms)
-6x -14 + 14 = 4 + 14   This will eliminate -14
-6x = 18      Now divide both sides by -6
x = -3
y = 2(-3)+14 = 8
Solution to the system is (-3, 8)

ANSWER TO YOU QUESTION: you can substitute (2x + 24) for y 

21, i'm pretty sure it's right lol