\(The\ area\ of\ the\ triangle:A_\Delta=\frac{b\cdot h}{2}\\----------------------\\A_\Delta=36\ u^2;\ h=9\ u\\\\subtitate\ to\ formula\ of\ the\ area\ of\ a\ triangle:\\\\36=\frac{b\cdot9}{2}\to\frac{9b}{2}=36\ \ \ \ |multiply\ both\ sides\ by\ 2\\\\9b=72\ \ \ \ |divide\ both\ sides\ by\ 9\\\\\boxed{b=8\ (u)}\)
