You convert ?mega? to ?kilo? by multiplying by 1000. So a ?7.5MW? transformer is the same as a ?7,500 KW? transformer.
But transformers are not normally described in terms of watts. Their ratings are generally given in terms of VA. Are you certain that you did not start with a ?7.5MVA? transformer?
If this is just an academic question, then the rest of the answer to your question is that you convert ?Watts? to ?VA? by dividing by the ?power factor.? Since you did not give us a value for power factor, I can only say that we don?t have enough information to solve the problem. You can assume a value if you like, perhaps 0.8 or 0.85 might do. But the person who posed this question to you will have to provide a value for power factor, or will have to allow you to assume a value, or else the problem cannot be solved.