x平方(2x)3次方-x(3x+8x4次方)
=(x^2)(8x^3)-x(3x+8x^4) [x^2意思是X的二次方]
=(8x^5)-3x^2-8x^5
=-3x^2
当x=2时,原式=-3×2^2=-3×4=-12.