ANSWER
3
EXPLANATION
We want to find the number that completes the problem:
42_ * 6 = 2538
To do this, we simply have to divide both sides by 6, so that we're left with the number 42_ on the left hand side:
[tex]\begin{gathered} \frac{42_{\text{\_}}\cdot\text{ 6}}{6}\text{ = }\frac{2538}{6} \\ 42_{}\text{ \_ = 423} \end{gathered}[/tex]By comparing, we see that the missing number is 3.