It has told you what you need, you need to state the objective function, the constrants and then graph them and solve the problem graphically.

The objective function is the function you want to maximise or minimise... In this case, you wish to minimise the cost.

If you call s the number of small tiles and l the number of larger tiles, since you know small tiles cost $3.50 each, and larger tiles cost $4.50 each, then the cost function is

Now what are your constraints?