MLE for the scale of Gamma claim sizes

Insurance claim sizes (in thousands of dollars) are modeled as independent Gamma variables with a known shape $k = 2$ and unknown scale $\theta$, density

$f(x \mid \theta) = \frac{x^{k-1} e^{-x/\theta}}{\Gamma(k)\,\theta^{k}}, \qquad x > 0.$

A batch of $n = 12$ claims has total size $\sum_i x_i = 60$ (thousand dollars).

Derive the maximum likelihood estimator of the scale $\theta$ and compute it.