solve (x+1)2-20=0

One way to do this:

(x + 1)² - 20  =  0

Add 20 to both sides:

(x + 1)²  =  20

Find the square root of both sides:

√(x + 1)²  =  ±√20       (need to use the ± sign)

    (x + 1)  =  ±√20 

Subtract 1 from both sides:

         x  =  -1 ±√20 

Simplifying the radical:

         x  =  -1 ±2√5