exam Maths Puzzle Numerical Ability

Can I have some help finding the inverse function of f.
f(x)=1/(-10 x^2 + 6),text( )x>=0

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exam Other Question

Choose values of x, then simplify it correctly.
10x^2-30x+20/x^2-1
I did something wrong on that one, maybe someone could show me how it works. (?7)+(?1/7)