What formula do you use to find the missing side of a triangle side a 70 feet side b 40 feet what is side a

To find the third side of a triangle when given the other 2 sides, use Pythagorean Theorem

There is no such thing as a formula to give you the third side of a triangle when you

only know the lengths of two sides. That's not enough information to figure out the

length of the missing side. There are an infinite number of possibilities.

If side-a is 70-ft and side-b is 40-ft, then side-c can be anything between 30-ft and 110-ft.

In order to nail it down to a single definite value, you'd also need to

know the angle between side-a and side-b.

The Pythagorean Theorem is useful IF this is a right triangle. But you don't know if it is.