A plastic ruler is rubbed with wool and gains 3.6 × 10¹¹ electrons…

Step 1. The charge. $Q = Ne = 3.6 \times 10^{11} \times 1.6 \times 10^{-19}\ \text{C} = 5.76 \times 10^{-8}\ \text{C} = 57.6\ \text{nC}$.

Step 2. The mass. $\Delta m = Nm = 3.6 \times 10^{11} \times 9.1 \times 10^{-31}\ \text{kg} = 3.28 \times 10^{-19}\ \text{kg}$.

Step 3. The sign. The ruler gained electrons, so its charge is negative.