What substitution should be used to rewrite 4x12 – 5x6 – 14 = 0 as a quadratic equation?
u = x2
u = x3
u = x6
u = x12

The answer is 

4x12 – 5x6 – 14 = 0
this equation is equivalent to 4(x^6)² – 5(x^6) – 14 = 0
so if u=x^6
we have 4(u)² – 5(u) – 14 = 0, and it is a quadratic equation

so the answer is u = x6