Explanation:
it seems like you understood the law of cosine (extended Pythagoras for non-right-angled triangles) and applied it mostly correctly for a).
but you made a little typo : suddenly, in the second half of the formula you changed the side length BC from 83.1 to 81.3. but it is 83.1.
so, the correct calculation is
a² = 83.1² + 95.5² - 2×95.5×83.1×cos(101) =
= 16,025.86 - 15,872.1×cos(101) =
= 16,025.86 - -3,028.539456... =
= 19,054.39946...
a = sqrt(19,054.39946) = 138.037674...
the rounded result is the same (138 km), but just for your information, and also, I want to use the correct numbers for b)
for b) we need to know :
bearings are angles, measured clockwise from north (north would be 0°).
when ship A looks to station C, the bearing is 146°.
when ship A looks to ship B, the corresponding bearing is 146 - the triangle angle at A (because that is how far we have to turn from looking at C to looking at B).
so, now, we need to apply the law of cosine again. this time to get angle A, as we have now side a.
c² = a² + b² - 2ab×cos(C)
and we want to get C, means now (since you called the baseline "a" in the first part)
83.1² = 95.5² + a² - 2×a×95.5×cos(angle A)
6,905.61 = 9,120.25 + 19,054.39946... - 2×138.037674...×95.5×cos(angle A)
6,905.61 = 28,174.64946... - 26,365.19574...×cos(angle A)
-21,269.03946... = -26,365.19574...×cos(angle A)
cos(angle A) = -21,269.03946.../-26,365.19574...
= 0.806708953...
angle A = 36.22437773...°
therefore, the bearing from A to B is
146 - 36.22437773... = 109.7756223...°