This parabola (quadratic polynomial) has the roots x= -8 and x= 1  (where y is equal to zero).


Hence, the quadratic polynomial has the form  p(x) = a*(x-(-8))*(x-1) = a(x+8)*(x-1) with the unknown coefficient "a".


To determine the value of "a", use the condition/(the fact from the condition) that

p(2) = -20 = a(2+8)*(2-1) = a*10*1 = 10*a.


It gives you   a =  = -2.

and finally your polynomial has the form

p(x) = -2*(x+8)*(x-1).


You can transform it further to any form you wish.