Or:
Since the problem provides Angle-Side-Angle information, the Sine Law can be used to solve the first triangle:
C/sin c = B / sin b = A / sin a
where:
C is the length of the base line = 100'
c is the angle of the intersection of the bearings = 75
B is the length of the leg opposite observer B (observer A's vertical triangle base)
b is the angle to the base line of observer B's bearing
A is the length of the leg opposite observer A (observer B's vertical triangle base)
a is the angle to the base line of observer A's bearing
Solve for either A or B using the known values of C and c:
A = sina (C/sin(180-(a+b))
A = sin60 (100/sin75)
A = .8660 (100/.9659)
A = 89.6573