An ideal fluid of density 800 kgm $-$ 3, flows smoothly through a bent pipe (as shown in figure) that tapers in cross-sectional area from a to ${a \over 2}$. The pressure difference between the wide and narrow sections of pipe is 4100 Pa. At wider section, the velocity of fluid is ${{\sqrt x } \over 6}$ ms $-$ 1 for x = $\underline{\hspace{2cm}}$. (Given g = 10 ms $-$ 2)