How do you find the area of a rectangular base?

You question is does not make sense. The base is a straight line , NOT an area.

You either want the Area of the rectangle or the Length of the base.

The Area of a rectangle is found by multiplying the base (length) to the width(height).

A = lW (or BH)

Algebraically rearrange to find the base length

B = A/H ( That is area divided by height).