x = 31 => 30 = x-1
\(\Rightarrow C=x^6-\left(x-1\right)x^5-\left(x-1\right)x^4-\left(x-1\right)x^3-\left(x-1\right)x^2-\left(x-1\right)x+70\)
\(=x^6-\left(x^6-x^5\right)-\left(x^5-x^4\right)-\left(x^4-x^3\right)-\left(x^3-x^2\right)-\left(x^2-x\right)+70\)
\(=x+70=31+70=101\)