Add all the sides up to equal 30 in an equation because you are given the perimeter. The perimeter is the length around a two dimensional shape. This means the perimeter is the length of all the side lengths added together.
The answer is 21x-25 because all you have to do is multiply the 1/4 by the 84x and then multiply 1//4 by negative 100. A simpler way of explaining this is just that you need to distribute