a) There are 4 spaces: _ _ _ _
You must put M in one of the four spaces = 4 possible ways.
The remaining three spaces may be filled by: D,O,E,S,T,I = 6*5*4 = 120 possible ways.
4*120 = 480 possible ways.
Think of it as: there are 120 possible ways to choose 3 different letters from D,O,E,S,T,I...