Answer:
Sam has 29 crayons
Explanation:
This problem does not really need to set up an equation to solve. We'll give the answer by simple logic deduction and simple calculations.
We know Luca has 13 more crayons than Sam. We can say that Sam has 13 less crayons than Luca.
Now we are told that Luca has 42 crayons. Considering the above relation found for the number of crayons Sam has, we can assure Sam has 42-13=29 crayons.
Conclusion: Sam has 29 crayons