What is the base?
What is the height?
The area of the arch is:
$\displaystyle
A(a,c)=\int_{x=x_1}^{x_2} f(x)\ dx
$
where $\displaystyle x_1$ and $\displaystyle x_2$ are the roots of $\displaystyle f(x)=0$
Now show us what you have tried and where you are having difficulties.
CB