pls teach me how to start amd solve part (A). thank you very much
$\displaystyle OQ$ is the slant height of the cone.
The formula for the Total Surface Area of a cone is
$\displaystyle \textrm{TSA} = \pi r^2 + \pi r s$
where $\displaystyle r$ is the radius and $\displaystyle s$ is the slant height.
Note that $\displaystyle \pi r^2$ is the area of the circular base, so $\displaystyle \pi r s$ is the curved area.
Since you're told that the curved surface has area $\displaystyle 252 \pi \,\textrm{cm}^2$ and the radius $\displaystyle r = 14\,\textrm{cm}$, we have
$\displaystyle 252 \pi \,\textrm{cm}^2 = 14\pi\,\textrm{cm}\cdot s$
$\displaystyle s = \frac{252\pi\,\textrm{cm}^2}{14\pi\,\textrm{cm}}$
$\displaystyle s = 18\,\textrm{cm}$.
You can find the height $\displaystyle OP$ using the Pythagorean Theorem.