Explanation:
Part 1: How to calculate the width
1-

This is pythagoras theorem. the slanted side is C so fill it in with 89 and the right side can either be a or b; let's say b. now fill this in.
2-

Now work out the squares (squaring means multiplying a number by itself)
2-

3- a2=7921-1521 work out the subtraction (a2 means a squared)
4- a2= 6400 Now take the root of that because you want to know a not a squared.
5-

A equals 80
Part 2: calculating area
1- the formula for the area of a triangle is area= 1/2 × length x width
2- 1/2 × 39 x 80= 3120 cm2 ( cm squared)