Polynomial $x(7-x)=7x-x^2$

This is the maximum value of the polynomial $\color{#D61F06}{x(7-x)=7x-x^2}$ for real $\color{#20A900}{x}.$ Since this is a quadratic curve, its turning point occurs in the middle of its two roots, which are $\color{#20A900}{0}$ and $\color{#20A900}{7.}$ Thus, it reaches its maximum when $\color{#EC7300}{x=3.5,}$ meaning that $\color{#EC7300}{3.5}$ and $\color{limegreen}{7-3.5=3.5}$ are the two real numbers you need.

If y = 7x - x^2, y' = 7 - 2x =0, and x = 3.5, x^2 = 12.25