How to round the next nearest dollar

An airplane travels at an unknown elevation before it descends 3 miles to an elevation of 15 miles. find the elevation of the plane before its descen.

If f(x) = 4x - 3 and g(x) = x + 4, find (f - g) (x)

What is the missing justification in the proof of the pythagorean identity below, sin^2x+cos^2x=1, using a right triangle?