How do you multiply a radical times a radical?

radical[(-7)*(-105)] = radical(7*105) = 7*radical(15) = 27.11 (approx).

√18

sqrt(3) x sqrt(15) = sqrt( 3 x 15 ) = sqrt( 45 ) = sqrt( 9 x 5 ) = 3 sqrt(5)

You can multiply the radicands together if the radical is the same. So, the answer is radical 13*17=radical 221

20