[tex]\\ \tt\rightarrowtail \vec{R}=\sqrt{(\vec{A})^2+(\vec{B})^2+2ABcos\theta}[/tex]
[tex]\\ \tt\rightarrowtail \vec{R}=\sqrt{(70)^2+(70)^2+2(140)cos57}[/tex]
[tex]\\ \tt\rightarrowtail \vec{R}=\sqrt{9800+280(0.54)}[/tex]
[tex]\\ \tt\rightarrowtail \vec{R}=\sqrt{9951.2}[/tex]
[tex]\\ \tt\rightarrowtail \vec{R}=99.7N[/tex]