The fact that they want a variable expression means that they don't want a specific answer to the problem. The want an expression with an unknown (like x or y) that they can plug in a number if they wanted to solve an equation...
1. The cost of y roses. Well it tells us that the roses cost $1.60 each. So if we bought 1 we would multiply it by 1.60 to find the cost. If we bought 50 we would multiply 50 x 1.60. Since the number of roses can change we call it a variable and we can replace it with x or y (or anything else for that matter). Since we have a variable, we can put whatever number we want into it to solve the equation. So to get back to your problem, the cost of y roses would be...
$1.60y that just means it will cost 1.60 x number of roses
2. Cost of 12 carnations. In this case we know how much the carnations cost and how many we buy so we just multiply...
12 x 1.15 = $13.80
3. The cost of m roses and f carnations That's "some number" of roses and "some number" of carnations. Roses are $1.60 so $1.60m and carnations are $1.15 so $1.15f. We are buying both so add them together...
