In a study comparing the mean performance scores of two independent groups, a researcher calculates a pooled variance $s_p^2 = 144$ for a sample of $n_1 = 12$ and $n_2 = 14$. If the group means are $\bar{x}_1 = 78$ and $\bar{x}_2 = 72$, what is the calculated $t$-statistic for the null hypothesis $H_0: \mu_1 - \mu_2 = 0$?