You know how a puppy is a kind of dog, but not all dogs are puppies? Well, the same thing is true for lots of other categories of things, including squares and rectangles.
The sum of the inner angles of a polygon with n sides is 
Since your polygon has 6 sides, the sum of its inner angles has to be

On the other hand, the sum of its inner angles is

And it has to be 720, so we have

Equate the expressions for m.
7x = 3x + 20
4x = 20 . . . . . . . subtract 3x
x = 20/4 = 5 . . . divide by 4
The value of x is 5.
From 55 mph to 70 mph is a 36% increase.
I hope this helped!
The answer is D. all you have to do is solve for y.