I agree with your answer of 3.88 for part A (but I don't know that you worked it correctly since you didn't show any work). Part B is done this way.
HAc = CH3COOH = acetic acid.
.................HAc ==> H^+ + Ac^-
initial...........0.1M... .0......0
change............-x......+x......+x
final............0.1-x.....x.......x
Ka = (H^+)(Ac^-)/(HAc)
Plug into Ka expession from the ICE chart I constructed above and solve for (H^+), then convert to pH.
Consider a weak acid-strong base titration in which 25.0 mL of 0.100 M acetic acid is titrated with 0.100 M NaOH.
a) Calculate the pH after the addition of 3.00mL of NaOH.
b) What is the pH of the solution before the addition of NaOH (pKa of acetic acid=4.75)
I got pH= 3.88 for part A
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