What does a variable represent? Pls help

Aboat's value over time is given as the function f(x) and graphed below. use a(x) = 400(b)x + 0 as the parent function. which graph shows the boat's value increasing at a rate of 25% per year?

If x-1 and x+3 are the factors of x3+10x2+ax+b find a and b

Which point-slope form equation represents a line that passes through (3,-2) with a slope of -4/5