Hi I cant seem to solve this equation. Wikipedia said to sub $\displaystyle z=x^2$ but I don't know how to then get the x results back. Once i have done this I get confused as to how to put it back into two quadratics which is what i've been aed to do.

Q Express $\displaystyle x^4-4x^2+16$ in the form $\displaystyle (x^2+Ax+B)(x^2+Cx+D)$

thnx.