we are told that x should represent the number of $1 increases in price of the tshirts.
each $1 increase = decrease in the number of shirts sold by 7
if the price increases once, that is a $1 increase and the number of shirts decreases by 7
if it increases by 2, that is a $2 increase and the number of shirts would decrease by 2 times 7 = 14
if it decreases x times, the number of shirts would decrease by 7x
Therefore, the expression for the revenue would be
(12 + x)(250 - 7x)