Question 636561
Let 
the height = x 
Base = x-8

Formula
Area= 1/2(height *base)

Solution 

1/2{x(x-8)}=42
x(x-8)=84
x^2-8x-84=0
x^2 -14x+6x-84=0
(x+6)(x-14)=0
x= -6,14

We'll take positive one
Base=x=14cm
Height =x-8=6cm