Two forces, f1 and f2, are acting on a body, as shown in the figure. the resultant force, r, is
newtons.

The exterior angle of 50 degrees is adjacent to angle ACB
(angle ACB) + 50 = 180
(angle ACB) + 50-50 = 180-50
angle ACB = 130 degrees

Using the law of cosines, we can say...
c^2 = a^2+b^2 - 2*a*b*cos(C)
R^2 = 38^2+40^2 - 2*38*40*cos(130)
R^2 = 4998.07433344708
sqrt(R^2) = sqrt(4998.07433344708)
R = 70.6970602885798

So the resultant force is approximately 70.6970602885798 newtons.

The answer to your question is d 68%