Perimeter is the sum of twice the width and twice the length of the rectangle. And since the question is asking us for an equation, we need to write, Area = 2(5x - 4) + 2(5x). When you are required to find the value of x with a given area, just plug that area into the equation, distribute, and simplify to get x.
There are two variables here, which are time (minutes) and the number of beats, and they both have a relationship (68 beats in one minute), therefore it is an example of a unit rate
