Area Under the Curves

Let S($\alpha$) = {(x, y) : y² $\le$ x, 0 $\le$ x $\le$ $\alpha$} and A($\alpha$) is area of the region S($\alpha$). If for a $\lambda$, 0 < $\lambda$ < 4, A($\lambda$) : A(4) = 2 : 5, then $\lambda$ equals