book Maths Puzzle Numerical Ability

i need some one to give the steps one by one for this problem.

f(x)=2x^2-x^4+5x^2-x if f(x)=3

i'm using the synthetic substituion of polynomials

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book Other Question

Can you help me subtract:

13/15 - 11/20

Thanks!
0.9872^X=0.5

how do i figure out what X is without just putting in numbers and seeing if they equal 0.5?