Can someone teach me how to do this

Let Width = X

So the Length is X +8  ( 8 feet more then the width)

Area of a rectangle is Length x width

Area of pool = 240 square feet

So now we have (x+8) * x = 240
Distribute the left side to get:

x^2 +8x = 240

Subtract 240 from each side:

x^2 + 8x - 240 = 0

Now Factor the polynomial.

(x-12) (x+20) = 0

12-12 = 0
-20 +20 = 0

So x is 12 and -20

since the dimension cannot be a negative number, x = 12
 
 The width = x = 12 feet

The length = the width  +8 = 12 +8 = 20 feet

The answer to your question is the last one

Google it that might work