How do you simplify #sqrt15 times sqrt21#?
When multiplying 2 square roots they can be combined into one.
Write each number as the product of its prime factors.
When simplifying roots of natural numbers is is often useful to first express the numbers as the product of prime numbers.
Same thing but with a slight twist in approach
Look for common factors. Notice that 3 will divide into both 15 and 21 with no remainder.
You can split these in the following way: