Question 443371
The altitude (AKA height) can be represented as b-5. The base is b. 

Recall that the formula for area is {{{A = (1/2)b*h}}}
{{{42 = (1/2) * b * (b-5)}}}}
{{{84 = b^2 -5b}}}
{{{b^2-5b -84 = 0}}}

Let's factor this:

What adds to -5 and multiplies to -84?

-12 and 7.

So we can factor this as (b-12)(b+7)

So b =-7, b = 12

A negative length doesn't make sense so...

The base is 12.