(i) If the cost function is C, then the average cost function is C/X. So in your example where C = x^2/25 + 3x + 200, the average cost (AC) will be C/X = AC = (x^2/25 + 3x + 200)/X = x/25 + 3 + 200/x

(ii) To find the marginal cost function you have to differentiate the cost function with respect to x. This will yield 2x/25 + 3.

(iii) the AC at 25 units: 25/25 + 3 + 200/25 = 1 + 3 + 8 = 12. The MC at 25 units: 2*25/25 + 3 = 2 + 3 = 5.

Regards.