Vector Calculus · Medium problem

A gas has velocity field $\mathbf F=(x^3,y^3)$ (position in meters, velocity in m/s). For a square tank occupying $-L\le x\le L$, $-L\le y\le L$, the total outward flow of gas across the tank's boundary, per unit time, is $128$ m$^3$/s. Find $L$.