I'm struggling with the following question for long. I tried to apply isoperimetric inequality $4\pi A\le {L}^{2}$, but my attempt has been unsuccessful. Could anyone give me a hint?

Let $AB$ be a segment of straight line and let $l>$ length of $AB$. Show that the curve $C$ joining $A$ and $B$, with length $l$, and such that together with $AB$ bounds the largest possible area is an arc of a circle passing through $A$ and $B$.

Let $AB$ be a segment of straight line and let $l>$ length of $AB$. Show that the curve $C$ joining $A$ and $B$, with length $l$, and such that together with $AB$ bounds the largest possible area is an arc of a circle passing through $A$ and $B$.