Find the magnitude of two forces f1 and f2 such that at right-angle to each other, their resultant is √13N. Also, if the two forces act at an angle of 60° to each other, their resultant is √19N

This can be solved a couple of ways. One way is to use the Pythagorean theorem to write equations for the magnitude from the components of the forces. That is what was done in the graph here.

Another way is to use the Law of Cosines, which lets you make direct use of the angle between the vectors.

... 13 = a^2 + b^2 - 2ab*cos (90°)

... 19 = a^2 + b^2 - 2ab*cos (120°)

Subtracting the first equation from the second, we have

... 6 = - 2ab*cos (120°)

... ab = 6

Substituting this into the first equation, we have

... 13 = a^2 + (6/a) ^2

... a^4 - 13a^2 + 36 = 0

... (a^2 - 9) (a^2 - 4) = 0

... a = ±3 or ±2

The magnitudes of the two forces are 2N and 3N, in no particular order.