We use the law of Cosines, resultant force
[tex]F =\sqrt{(F_{1})^2 + (F_{2})^2 + 2 F_{1} F_{2} cos \theta }[/tex]
Here, [tex](F_{1})[/tex] and [tex](F_{2})[/tex] are forces acting at angle [tex]\theta[/tex] with each other.
Given [tex]F_{1} = 80 N[/tex], [tex]F_{2} = 100 N[/tex] and [tex]\theta = 60^0[/tex].
Substituting these given values in above formula we get
[tex]F =\sqrt{(80)^2 + (100)^2 + 2 \times 80 \times 100 \ cos 60^0 } = 156. 20 \ N[/tex].
Thus, the resultant force is 156 N.
