Let x be the number of $0.25 decreases in price. The number, N, of sandwiches sold is:
N = 330 + 15x
The price per sandwich, P, is:
P = P0 - 0.25x
where P0 is the full price (when x = 0). I suspect the problem tells you this number, so just fill in the value.
The Revenue, R, is the number of sandwiches sold times the price per sandwich:
R = N·P
Substitute in the expressions for N and P then multiply it out. You'll get a quadratic equation with a negative coefficient for the x2 term. This means the parabola is inverted and the vertex is the maximum revenue, so you'll need to find the coordinates of the vertex. The x value of the vertex is the number of $0.25 decreases that yields the maximum revenue and the y value is the maximum revenue. Plug the x value of the vertex into the price formula above to get the price per sandwich.