Answer:
3x^4+9x^3
Explanation:
Perimeter of a square is expressed as;
P = 4L
L is the length of each side
Given
P = 12x^4 + 36x^3
12x^4 + 36x^3 = 4L
4(3x^4+9x^3) = 4L
Divide both sides by 4
4(3x^4+9x^3)/4 = 4L/4
(3x^4+9x^3) = L
L = 3x^4+9x^3
Hence the length of each side is 3x^4+9x^3