When it says to factor it completely, it means to find a factor for each of those three numbers, but a common factor. A common factor is a factor that all three numbers share.
And the largest number that is a similar factor of 24x, 28y AND 52 is 4. And now that you know the similar factor between them, you have to put it in an expression showing it. Your final answer would be :
4(6x + 7y + 13)
This would be your final answer because, okay, let's check. The number outside the parentheses goes to everything inside the parentheses, because the Distributive Property. 4 and 6 make 24, that makes it 24x. 7 and 4 make 28, that becomes 28y. And lastly, 13 and 4 make 52, so when this is all distributed and everything, it becomes what it was before : 24z + 28y + 52
But your answer is 4(6x + 7y + 13)
