Book Maths Puzzle Algebra

how do you do::

36 - 49p^2 = 0

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Book Other Question

How do I solve for x for:
Volume = (.512-2*x)*(.622-2*x)*x = 0.0144144 cubic metres
I know that x=0.1 metres high, where brackets are width and depth resp. but how?
A carpenter works out the number of nails he needs(n)by dividing the length in centimetres(l) of each board by 20. Construct a formula which will help him work out the number of na