Answer
2.2 kg
Work Step by Step
Resultant force $F_{net}=\sqrt {F_{1}^{2}+F_{2}^{2}+2F_{1}F_{2}\cos\theta}$
$=\sqrt {(20\,N)^{2}+(35\,N)^{2}+2\times20\,N\times35\,N\times\cos 80^{\circ}}$
$=43\,N$
Now, $F_{net}=ma$ (Newton's second law)
$\implies m=\frac{F_{net}}{a}=\frac{43\,N}{20\,m/s^{2}}=2.2\,kg$
