first to solve this you must multiply a negative and a positive equals a negative: (-) x (+) = (-). for your problem, it will look like the following:
(-2√15) x (4√21)
then you will need to calculate the product
-8√315
next you will need to simplify the radical
-24√35
this means that your final answer is -24√35
let me know if you have any further questions
:)