The altitude of a triangle is 5m less than its base the area of the triangle is 42 find the base

A=(1/2)bh
h=height=altitude
h=b-5

area=42
42=(1/2)bh
times 2
84=bh
sub b-5 for h
84=b(b-5)
84=b²-5b
minus 84
0=b²-5b-84
factor
what 2 numbres multiply to get -84 and add to get -5
-12 and 7
0=(b-12)(b+7)
set to zero
b-12=0
b=12

b+7=0
b=-7, false, can't have negative measures

so

b=12
the base is 12m