let √ be a square root
3√(5c)×7√(15c²)
1. take out the c² from√
3√(5c)×7c√15
2.multiply the coefficients and √, also use the √ rule of using single√.
21c√(5c×15)
3. and 4. get 25 of the √.
21c√(25×3c)
21×5c√(3c)
5. multiply the coefficients
105c√(3c)