Say we have $z^{[L]} = \begin{bmatrix} 2 -1 3 7 \end{bmatrix}$. Then, $t = \begin{bmatrix} e^2 e^{-1} e^3 e^7 \end{bmatrix} \approx \begin{bmatrix} 7.39 0.37 20.1 1096.6 \end{bmatrix}$. We sum the entries in $t$ to yield the denominator of the activation: $7.39 + 0.37 + 20.1 + 1096.6 = 1124.46$. Finally, normalize the probabilities by dividing each entry in $t$ by the summation we just computed: $a^{[L]} = \begin{bmatrix} 7.39/1124.46 0.37/1124.46 20.1/1124.46 1096.6/1124.46 \end{bmatrix} \approx \begin{bmatrix} 0.007 0.0003 0.018 0.975 \end{bmatrix}$.