bit messy, making it look nice will waste time
A = 98x - \frac{21x^2}{2} \\ \frac{dA}{dx} = 98 - 21x \\ $At $\frac{dA}{dx}=0,$ stationary points exist \\ \\98 - 21x = 0, x= $\frac{14}{3} \\ \\ $Test for max or min \\ $\frac{d^2A}{dx^2} = -21 \\ $Since $\frac{d^2A}{dx^2} <0, $then for all...