a$.$ Display the possible errors and solutions for cell $B2,$ and then convert it to a number.

b$.$ Trace the precedents to the formula in cell $H7,$ which should multiply the optional extra payments by the scheduled number of payments. Correct the error.

$6.$ Draw attention to the optional extra payments in the range B$10$:D$10$ by adding a thick outside border using the Tan, Accent $4$ shape outline color.