Factor each expression

45c + 10d

The greatest common factor of 45 and 10 is 5
The value 5 is the largest factor shared in common between 45 and 10

We can factor each monomial as follows
45c = 5*9c
10d = 5*2d

Which allows us to say
45c + 10d = 5*9c  +5*2d
45c + 10d = 5*(9c+2d)
Notice how I used the distributive property in reverse. If we distribute the 5 back in, we get 45c+10d again

5(9c+2d)