$$ \begin{cases} F_1 &= 23\ut{N}\\ F_2 &= 38\ut{N}\\ \theta &= 80\degree\\ a&= 15\ut{m/s^2} \end{cases} $$ $$ \begin{aligned} &\text{Law of cosines,}\\ c^2&=a^2+b^2-2ab\cos\theta,\\ {F_S}^2 &= {F_1}^2+{F_2}^2-2 F_1 F_2 \cos(\pi-\theta) \\ &={F_1}^2+{F_2}^2+2F_1F_2\cos\theta\\ \end{aligned} $$ $$ \begin{aligned} F_S&=\sqrt{{F_1}^2+{F_2}^2+2F_1F_2\cos\theta}\\ &=\sqrt{{23}^2+{38}^2+2\cdot23\cdot38..