Hello, thank you for taking the time to post your question!
The greatest common monomial factor (GCMF) means that you want to find the largest number that divides into both terms of the expression.
In this case that would be 2, making the factored expression
2(a + 5b)
I hope that helps you get moving in a better direction on this type of question! Feel free to reach out if you have any additional questions beyond that :)