How I start and tell me step by step and with the hall area with answer

Break this shape up into two sections: the triangle and the semicircle. First we will find the area of the triangle using the following formula:
[tex]\: \frac{1}{2} (base \times height)[/tex]
From the problem, one side of the triangle (height) is 21 m. The other side of the triangle (base) goes from the edge of the semicircle to the center, so it will be equal to the semicircle's radius of 10 m. Plug these into the formula.
[tex] \frac{1}{2} ( \: b \times h) \: = \frac{1}{2} (21 \times 10) = 105{m}^{2} [/tex]
Now find the area of the semicircle. The area of a normal circle is as follows:
[tex]\pi {r}^{2} [/tex]
The radius of our circle is 10m. Since it exactly half of a normal circle, simply half the area you would get for the full sized circle.
[tex] \frac{1}{2} \pi {r}^{2} = \frac{1}{2} \pi {10}^{2} = 157 {m}^{2} [/tex]
Add the two areas together to get the final area.
[tex]105 {m}^{2} + 157 {m}^{2} = 262 {m}^{2} [/tex]