Step-by-step explanation:
(3x+20)² is the same as
(3x+20)(3x+20)
you need to have for different numbers
SO
3x*3x is 9x²
3x*20 is 60x
20*3x is also 60x
20*20 is 400
put it together
9x²+60x+60x+400
simplify to
9x²+120x+400
