Fine the surface area of cylinder whose diameter is 21cm and the hight is 7cm

The diameter ($d$) is 21 cm. The radius is half of the diameter:

$$r = \frac{d}{2}$$ $$r = \frac{21}{2}$$ $$r = 10.5 \text{ cm}$$

The formula for the surface area of a cylinder is $A = 2\pi r^2 + 2\pi r h$, where $r$ is the radius and $h$ is the height.
Substitute the values $r = 10.5$ cm and $h = 7$ cm into the formula:

$$A = 2\pi (10.5)^2 + 2\pi (10.5)(7)$$ $$A = 2\pi (110.25) + 2\pi (73.5)$$ $$A = 220.5\pi + 147\pi$$ $$A = 367.5\pi \text{ cm}^2$$

The surface area of the cylinder is $367.5\pi \text{ cm}^2$.