The greatest common factor of these two terms is 5cd.
In order to find this we first need to look at all the factors of the two coefficients.
-10: -1, 1, 2, 5, 10
15: 1, 3, 5, 15
The greatest number that is in both is 5. So we use that as the coefficient. Next we can only take out a variable in the amount that it is at its least. Since we have only 1 c in the second term, we can only use one c. Since we have only one d in the first term, we can use only one d.
This gives us 5cd