Three forces 3N, 4N and F keep an object in equilibrium . If the angle between 3N and 4N force is ${60}^{\mathrm{o}}$. The value of F(in N) is 

Magnitude of F must be equal to the magnitude of the resultant of 3N and 4N forces; but F and the resultant of 3N and 4N forces are oppositely directed 

$\therefore \mathrm{F}=\sqrt{3^2+4^2+2.3.4.\cos {60}^{\mathrm{o}}}\mathrm{N}=\sqrt{37}\mathrm{N}$