I am stuck on this math question and want to know how to answer it

Question

A backyard has dimensions of 40 `m` by 30 `m`. A patio is to be constructed in one corner of the backyard with a uniform width of grass around the patio. If the area of the patio is 875 `m^{2}`, what are the dimensions of the patio?

Answer 1

back yard has area = 40(30) = 1200

patio has area 875

grass has area 325 = 4w^2 + 2w(30-2w)(2w)(40-2w)

16w^4-560w^3+4804w^2 -325 =0

graph it and look for the x intercepts

4 solutions but 3 are negative, only positive is about 2.75

patio dimensions are about 40-2(2.75) by 30-2(2.75)

= about 34.4958 ft by 24.4958 ft