How much will it weigh by then? (300 + 8x pounds)
What will the price per pound be by then? (In cents, 150 – 2x ¢)
How much will you get for selling it? (Multiply the number of pounds by the price per pound.)
How much will you have had to pay for the steer plus the cost of keeping it? (In cents, 300 times 150 for the original cost, plus 100x for the cost of keeping it.)
So how much will the profit be? (Income from sale, less total costs.)
Finally, you have a formula for the profit (in terms of x). Now the problem has become purely mathematical, and you just have to figure out what its maximum value is.