Explanation:
the way I understand this :
the equipment has to be bought, no matter how many players there are. these equipment costs are a fixed expense.
the costs of the uniforms depends on the number of players.
a)
x = number of players (uniforms)
cost(x) = 25x + 612
because we have to pay the $612 in any case, plus then $25 per player for the uniform.
b)
they paid $912.
so,
912 = 25x + 612
25x = 300
x = 300/25 = 12
they bought 12 uniforms (for 12 players).