You are given an expression for y, so put that expression where y is in the first equation.
... 12x -5(x+4) = -20 . . . . . x+4 substituted for y
... 12x -5x -20 = -20 . . . . eliminate parentheses
... 7x = 0 . . . . . . . . . . . . . . collect terms, add 20
... x = 0 . . . . . . . . . . . . . . divide by 7
... y = x +4 = 0 +4 = 4 . . . put the value of x into the expression for y
The solution is x=0, y=4.