I need help with number 3, someone please help me.

Answer:
B
Step-by-step explanation:
By finding x we will know the triangle is the isosceles triangle. So both high and base are a.
So the area will be:
[tex] \frac{h \times b}{2} \: \: so \: \: \: \frac{ a\times a}{2} = \frac{ {a}^{2} }{2} [/tex]