Assuming x is the number of weeks, the equation would be 490 - 10x = 370. Starting with $490 and subtracting ten multiplied by the number of weeks will get the amount.
F(x)=x^2+3 has a domain of X> or equal to 3, and a range of all real numbers I'm not too sure about the inverse but I think the equation is sqrt(x-3)=y