Answer: 5(a-5d)
Step-by-step explanation:
The GCF (the largest number that can be factored evenly out of the expression is 5. Thus, we divide out 5 from the expression, leaving a and -5d inside of the paranthesis.
We can check our work by redistributing 5 to the factor within the parenthesis. This will prove whether or not your factoring is correct. In this case, taking out the 5 is the correct factorization.