Answer
a$\sqrt s^{2} - a^{2}$
Work Step by Step
Using Heron's formula if three sides of a triangle have length a,b,c then area A of a triangle is
A = $\sqrt s(s-a)(s-b)(s-c)$
where s = $\frac{a+b+c}{2}$
Lets take a=QM = s, b=QN = s and c=MN = 2a
s = $\frac{s+s+2a}{2}$
= $\frac{2s+2a}{2}$ =s+a
A = $\sqrt (s+a)(s+a-s)(s+a-s)(s+a-2a)$
=$\sqrt (s+a)*a*a*(s-a)$
=a$\sqrt s^{2} - a^{2}$ (Since (a+b)(a-b) = $a^{2}$ - $b^{2}$)
