Solve each system by substitution y=-5 5x+4y=-20

If you mean what is x if y=-5 so you should just do like:
5 x + 4 ( -5 ) = -20 ⇒ 5 x -20 = -20 ⇒5 x = 0 ⇒ x=0 :))
i hope this be helpful
* merry Christmas * 

In substitution, we need to replace the value of the variable and them solve.

y= -5

5x + 4(-5) = -20
5x + -20 = -20

Now do the inverse operation.

5x - 20 = -20
     +20    +20

5x = 0
5x/5 = 0/5 
o divided by 5 is o.

x= 0  and y = -5