My work so far :

R = –25n2 +250n +5000 convert to the vertex form

R= -25(n\^2-10n)+5000

R=-25(n\^2-10n+25-25)+5000

R= -25((n-5)\^2 -25) +5000

R= -25(n-5)\^2+625+5000

R= -25(n-5)\^2+5625

n = 5

Max revenue = -25(5-5)\^2+5625

5625 is the max revenue

Now how can i find the selling t shirt price