I would interpret "down by 10%" to be times 0.9, and not a percentage point change, so e.g. 40% to 36%, not 40% to 30%. There are many ways to do the algebra, here is one:

You have total demand D = o + c + 450w. Here o and c are the demand fulfilled by oil/coal directly, w is the demand fulfilled by a single windmill. You know that o = 0.4D, c = 0.45D. Subtract those to get 0.15D = 450w, or w = 0.15D/450 = D/3000.

Next year, D' = 1.2D, o' = 0.9o, c' = 0.92c. Windmills fulfill the same demand each, so w' = w. You have D' = o' + c' + xw'. Substitute everything to get 1.2D = 0.9 \* 0.4D + 0.92 \* 0.45D + xD/3000. Solving this will give you the number of windmills needed next year x, then subtract 450 to get your answer.