Does anybody knows how it's done?
I give them stars and crown

Write both vectors in component form:

A = (12.0 u) (cos(30°) i + sin(30°) j ) ≈ (10.4 i + 6.00 j ) u

B = (8.00 u) (cos(80°) i + sin(80°) j ) ≈ (1.39 i + 7.88 j ) u

Subtract B from A :

A - B ≈ (9.00 i - 1.88 j )

(a) The magnitude of this vector is

||A - B|| ≈ √(9.00² + (-1.88 )²) u ≈ 9.20 u

(b) This vector has direction θ such that

tan(θ) ≈ (-1.88 u)/(9.00 u) ≈ -0.209

A - B has a positive x-component and a negative y-component, which means it's directed into the fourth quadrant, so

θ ≈ arctan(-0.209) ≈ -11.8°

(c) The negative sign in the angle found in part (b) indicates that A - B makes an angle of 11.8° below the positive x-axis.

f=ma newton's second law 7x8=56n

option number c is the right answer.