two vectors of equal magnitude have a resultant equal to either of them, then the angle between them will be equal to...? 

Hello!

The resultant (R) of two forces (A and B) is mathematically represented as

 R² = A² + B² + 2ABCosθ  

where θ is the angle between forces A and B. Now, as all three forces are equal, so

 let R = A = B = x

So, 

 x² = x² + x² + 2x.xCosθ

or  x² = 2x² (1 + Cosθ) 

 1/2 = 1 +  Cosθ

thus ,  Cosθ =  -1/2

 or θ = 120^o

The answer was calculated correctly by @prakharmilitary.

Hello @prakharmilitary ! good job, keep it going!