# The perimeter of a triangle is 7a - 11b. If two of its sides are 2a + b and a - 9b, what is the third side?

Mar 19, 2018

$4 a - \textcolor{w h i t e}{\text{d}} 3 b$

#### Explanation:

$2 a + \textcolor{w h i t e}{\text{d}} b$
ul(color(white)("d")a-9b larr" Add"
$\textcolor{red}{3 a - 8 b} \leftarrow \text{ Sum of 2 sides}$

$\textcolor{w h i t e}{\text{d}}$

$7 a - 11 b \leftarrow \text{ Sum of all 3 sides}$
ul(color(red)(3a-color(white)("d")8b)larr" Subtract")
4a-color(white)("d")3b larr" Remaining side"