To purchase $$14,400$ worth of restaurant equipment for her business, Susan made a down payment of $$1200$ and took out a business loan for the rest. After $2$ years of paying monthly payments of $$585\mathrm{.}04,$ she finally paid off the loan.

$($a$)$ What was the total amount Susan ended up paying for the equipment $($including the down payment and monthly payments$)?$

$($b$)$ How much interest did Susan pay on the loan?

$ $I$