\(\left(x-3\right)\left(x-2\right)\left(x+1\right)\left(x+2\right)=84\\ \Leftrightarrow\left[x\left(x-2\right)-3\left(x-2\right)\right]\left[x\left(x+2\right)+\left(x+2\right)\right]=84\\ \Leftrightarrow\left[x^2-2x-3x+6\right]\left[x^2+2x+x+2\right]=84\\ \Leftrightarrow x^2\left(x^2+3x+2\right)-5x\left(x^2+3x+2\right)+6\left(x^2+3x+2\right)=84\\ \Leftrightarrow x^4+3x^3+2x^2-5x^3-15x^2-10x+6x^2+18x+12=84\\ \Leftrightarrow x^4-7x^2-2x^3+8x=84-12=72\)