exam Maths Puzzle Numerical Ability

Problems like this one really confuse me. Please help.

m^3n-10m^2n^2+24mn^3

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exam Other Question

I asked this question once before, but I did not understand what the person did.
2x^2-18/4x^8 X 8x^4/6x^2+6x-36
I know you are suppose to factor it as low as you can and then red Walt made an extra $9000 last year from a part-time job. He invested part of the money at 9% and the rest at 8%. He made a total of $780 in interest. How much was invested at 8%?