we know that
The perimeter of a triangle is equal to the sum of its three length sides
we have
9x+10x^2-x^3
4x^2-7x+8
-x^2+9x+12
so
Sum the three length sides
P=(9x+10x^2-x^3)+(4x^2-7x+8)+(-x^2+9x+12)
Group terms
P=(-x^3)+(10x^2+4x^2-x^2)+(9x-7x+9x)+(8+12)
Combine like terms
P=-x^3+13x^2+11x+20
therefore
