We need to choose and define a variable.
Let the width of the border be
Now we can define the length and width of the mural, remembering that the border is on both sides.
Length of mural =
width of mural =
The area of the wall is
The area of the mural is 75% of this:
Now we have enough information to write an equation and solve it.
Using the formula:
Estimating which value we should use gives the following:
Obviously we cannot subtract 13 twice from either the length or the width, so the second answer is the one we need.