Answer
$w_a=102.5N$
Work Step by Step
Finding the rope's tension with the horizontal net force:
$F_{net} = 0$
$Tcos(\theta) -f_s = 0$
$Tcos(\theta) = f_s$
$Tcos(\theta) = μ_s \times F_N$
$T = \frac{cos(\theta)}{μ_s \times F_N}$
$T = \frac{0.25 \times 711N }{cos(30^{\circ})} = 205N$
Finding the weight of block a using the vertical net force:
$F_{net} = 0$
$Tsin(\theta) -w_a = 0$
$w_a=Tsin(\theta)$
$w_a=205N \times sin(30^{\circ})$
$w_a=102.5N$
