A company borrowed at $6\mathrm{.}78\%$ compounded monthly to purchase equipment, agreeing to make payments of $$2,590$ at the end of every three months for $12$ payments.

$($a$)$ What is the equivalent cash price of the equipment?

$($b$)$ How much will be owed at the end of two years?

$($c$)$ How much of the principal will be repaid within the first two years?

$($d$)$ How much interest is paid during the first two years?