Answer:
The length of the pen is 26.25 feet.
Explanation:
Given:
Blake will use part of a garage wall as one of the long sides of a rectangular dog pen. Blake wants the dog pen to be three times as long as it is wide.
The plan of Blake is to use 70 feet of fencing.
Now, to get the length of the dog pen.
Let the width of the pen be

So, the length of the pen be

As, Blake plans to use 70 feet of fencing.
So, the pen's perimeter = 70 feet.
Now, to get the length we use formula:

Dividing both sides by 8 we get:

Then, to substitute the value of
:

Therefore, the length of the pen is 26.25 feet.