A uniform disc of mass 3.0 kg rolls without slipping down a slope at 35°…

Step 1. Acceleration. $a = \tfrac23 \times 9.8\ \text{m/s}^2 \times \sin 35^\circ = 3.747\ \text{m/s}^2$.

Step 2. Friction. $f = mg\sin\theta - ma = 16.86\ \text{N} - 11.24\ \text{N} = 5.621\ \text{N}$, up the slope.

Step 3. Coefficient. $\mu \ge \dfrac{5.621}{3.0 \times 9.8 \times \cos 35^\circ} = 0.2334$, which is $\dfrac{\tan 35^\circ}{3}$.