Erika is weighing her options for transportation. She's narrowed them down to the following:

Option A: buying a new car, requiring a loan to finance $$18,000$ at $1\mathrm{.}9\%$ interest compounded monthly over $60$ months.

Option B: buying a $6-$year old, used car, requiring a loan to finance $$6,000$ at $3\mathrm{.}6\%$ interest compounded monthly over $48$ months.

The formula for calculating the amount of a loan payment is:

$PMT=\frac{P(\frac{\text{A}\text{P}\text{R}}{n})}{[1-(1+\frac{\text{A}\text{P}\text{R}}{n}{)}^{-nY}]}$

P: Loan principal

$n$ : number of yearly compounding$/$payment periods APR: Annual Percentage interest Rate $Y$ : length of the loan, in years

a$.$ Calculate Erika's monthly payment for Option A$.$

b$.$ Calculate Erika's monthly payment for Option B$.$