This is an algebra problem that involves finding the expression for the perimeter of a square given its area and side length. According to ¹, the perimeter of a square is equal to 4 times the length of its side, or $$P = 4s$$
If the area of a square is given by $$A = 9x^2 + 60x + 100$$ and the side length is given by $$s = mx + n$$ then we can find the perimeter by substituting s into the formula above, or $$P = 4(mx + n)$$
To simplify this expression, we can use the distributive property of multiplication, or $$P = 4mx + 4n$$
This is the expression for the perimeter of a square in terms of m, n and x.
To find the perimeter when x = 7 feet, we can plug in this value into the expression above, or $$P = 4m(7) + 4n$$
To simplify this expression, we can multiply 4 and 7, or $$P = 28m + 4n$$
This is the perimeter of a square when x = 7 feet in terms of m and n