If the circumference of the circle with the radius $21 \mathrm{c}\mathrm{m}$ is $k \mathrm{cm}$, then find the value of $k.$ (Take $\mathrm{\pi }=\frac{22}{7}$)

If the area of the circle whose radius is $5 \mathrm{cm}$ is $k {\mathrm{cm}}^2$, then find the value of $k$ (correct up to two decimal places). (Take $\mathrm{\pi }=\frac{22}{7}$)

From a circular sheet of radius $4 \mathrm{c}\mathrm{m},$ a circle of radius $3 \mathrm{c}\mathrm{m}$ is removed. If the area of the remaining sheet is $k {\mathrm{cm}}^2$, find the value of $k.$ (Take $\mathrm{\pi }=3.14$)

Find $k$, if the perimeter of given semicircle is $k \mathrm{cm}$ (correct up to two decimal places).