exam Maths Puzzle Numerical Ability

Can you please help me with this. Can you please explain this step by step so i can understand it.

The heights of 18 year old men are approximately normally distributed, with

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exam Other Question
I dont know how to solve these problems completely.
1. 20x^4-12x^3+8x^2

2.6x^2+2x-20

3.5w^2-20

4.5y^4-405 x^2=25,other than trial and error,how does one solve for x