Using the inclusion-exclusion principle:

$|\text{Choir} \cup \text{Band}| = |\text{Choir}| + |\text{Band}| - |\text{Both}|$

We know there are $26$ students total, $14$ in choir, and $18$ in band:

$26 = 14 + 18 - |\text{Both}|$

$|\text{Both}| = 14 + 18 - 26 = 6$