Another method to this same question is to use the quadratic equation.

x = [ -b + or - sqrt (b^2 - 4(ac) ] / 2a.

To do this, one would first set the overall equation to 0 by moving the 20 on the right over to the left side of the equation by subtracting it by both sides.

That gives you -4^2 - 24y - 20 = 0

a, b and c are the coefficients of the quadratic formula when they are in the order from highest exponent value to lowest (so y^2, y and the number alone).

In this case:
a = -4
b = -24
c = -20

Plugging these values into the quadratic formula would get the two solutions of -1 and -5.

To memorize the quadratic formula, many people set it to the tune of "pop goes the weasel" (aka the jack in the box song). My account isn't old enough to be able to upload videos, but this is easy to find with a quick web search.

For many problems (this one included), factoring is faster. I choose to use the quadratic formula because I'm the most familiar with it and not all problems are easy to factor like this. Either works!