Factoring - I always found this to be easy (one of the few things in math that I felt was easy!)
x^2 + 20x + 100 = 36
First step is to get the right side to zero. Do this by subtracting 36.
x^2 + 20x + 64 = 0
Now we have to factor the expression. We could use quadratic formula, completing the square, or use the product/sum rule if possible (which it is).
64's factors are:
1, 64
2, 32
4, 16
8, 8
We need the two factors that sum to 20...
(x + 4)(x + 16) = 0
Set each factor to zero
x + 4 = 0 and x + 16 = 0
Solve each equation
x = -4 and x = -16
You should always check for extraneous solutions. I recommend using desmos.com to check answers!