(a)
A = (75.0 cm) (cos(30°) i + sin(30°) j)
A = (75.0 cm) (√3/2 i + 1/2 j)
A ≈ (64.95 i + 37.5 j) cm
The x component is the coefficient of the i unit vector, while the y component is the coefficient of the j unit vector.
(b) "points along the negative axis" in other words means that B makes an angle of 180° with the positive x-axis in the counterclockwise direction. In particular this tells you that B is parallel to the x-axis, so its y component would be zero.
B = (25.0 cm) (cos(180°) i + sin(180°) j)
B = (25.0 cm) (-1 i + 0 j)
B = (-25.0 i) cm
(c) If the negative x-axis corresponds to 180°, then 45° below this would make an angle of 180° + 45° = 225° with the positive x-axis.
C = (40.0 cm) (cos(225°) i + sin(225°) j)
C = (40.0 cm) (-1/√2 i - 1/√2 j)
C ≈ (-28.28 i - 28.28 j) cm
(d) The sum A + B + C has components that are the sums of the components of each of A, B, and C.
A + B + C ≈ (11.67 i + 9.22 j) cm
(e) The magnitude of the vector sum is
||A + B + C|| = √((11.67 cm)² + (9.22 cm)²) ≈ 14.87 cm
(f) Since both components of the vector sum are positive, you know that it's a vector that terminates in the first quadrant of the x,y-plane, so it makes an angle with the positive x-axis of
arctan(9.22/11.67) ≈ 38.3°