[taylorwindsor81]

-x - 4y =16

[ferdilouw]

You've got 2 variables ( x and y ), therefore you need two equations to be able to solve for both x and y.

You can get x solved in terms of y like in:
x = -4y - 16

If you have another equation for x and y, you can substitute x with -4y - 16 and then solve for y.