Agriculture Dai and colleagues $^{16}$ created a mathematical model that showed the yield response of corn in Indiana to soil moisture was approximated by the equation $y=-2600+2400 x-70 x^{2}+\frac{2}{3} x^{3},$ where $y$ is measured in bushels per acre and $x$ is a soil moisture index. Find the value of the soil moisture index on the interval [0,40] that maximizes yield.