A proton moves along +x at 8.0 × 10⁵ m/s…

Step 1. The electric force. $F_E = eE = 1.6 \times 10^{-19}\ \text{C} \times 3.0 \times 10^{4}\ \text{V/m} = 4.8 \times 10^{-15}\ \text{N}$, along $+y$.

Step 2. The magnetic force. $F_B = evB = 1.6 \times 10^{-19}\ \text{C} \times 8.0 \times 10^{5}\ \text{m/s} \times 0.080\ \text{T} = 10.24 \times 10^{-15}\ \text{N}$. Its direction is that of $\hat{i} \times \hat{k} = -\hat{j}$, so it is along $-y$.

Step 3. The net force. The two forces are opposite, so $F = 10.24 \times 10^{-15} - 4.8 \times 10^{-15} = 5.44 \times 10^{-15}\ \text{N}$, along $-y$.