$1)$ Using:

$$ the algebraic expression of heat capacity,$=/,$

$$ the $1$st Law of Thermodynamics, $=+,$ and

$$ the Ideal Gas Law, $=,$

to find a symbolic expression for $$ in terms of $$ and other constants. Recall $=/.$

Using $1)$ solve for molar specific heat at constant pressure for a monatomic gas and compare to your result in $3).$