Question

What is measure of angle R?

Answer 1

You have three options, all of them work.
sin(angle) = opposite/hypotenuse
cos(angle) = adjacent/hypotenuse
tan(angle) = opposite/adjacent

You are already given the length of the lines, so all you can is to take the inverse of whatever you choose to get the angle.

$sin^{-1}$ (5/13) = 22.62

$cos^{-1}$ (12/13) = 22.62

$tan^{-1}$ (5/12) = 22.62