Given:
Perimeter of the triangle = 99 ft
Length of two sides are 20 ft and 42 ft.
To find:
The length of missing side.
Solution:
Let the missing side be x.
Perimeter of a triangle is the sum of all sides of that triangle.
![x+20+42=99](https://img.qammunity.org/2021/formulas/mathematics/high-school/igrt3hwrlrj934l56igna75uionzjg9xd1.png)
![x+62=99](https://img.qammunity.org/2021/formulas/mathematics/high-school/n46vtrq9tsy52707ngksmxfoorjj60sast.png)
Subtract 62 from both sides.
![x=99-62](https://img.qammunity.org/2021/formulas/mathematics/high-school/wwk5dv2kbzke5tgln26h0fq5fo0e4o7cj5.png)
![x=37](https://img.qammunity.org/2021/formulas/mathematics/college/wh4m3of1ad43n0ck9ajg0nzgscbnncymcn.png)
Therefore, the missing side of the triangle is 37 ft.