He height of a triangle is 9 m less than is base the area is 56 m^2 what is the length of the base?

Formula for area of triangle:
A= 1/2 bh
Where b is the base and h is the height of the triangle.
Here, h= b-9
A=56 m^2
Put values in formula.
56= 1/2 (b) (b-9)
56=b(b-9)/2
Multiply both sides by 2.
112=b^2-9b
or
b^2-9b-112=0
Factorise the equation.
b^2-16b+7b-112=0
b(b-16)+7(b-16)=0
(b+7)(b-16)=0
Using zero factor theorem.
b+7=0 or b=-7
and
b-16=0 or b=16
Ignore the value b=-7, because length cannot be negative.
So length of base=16 m

Base=16m