write the first three terms of the expansion of (a-b)^10
what are the last three terms of
(a-b)^7
Look up "Pascal's Triangle", you will find it very useful for these type of questions.
If not then ....
(a-b)^10
= a^10 + 10a^9(-b) + 10*9/2!a^8(-b)^2 +
= a^10 - 10a^9b + 45a^8b^2 + ...