When you multiply 2 "square root expressions", you can put the multiplication problem under 1 square root. So,
√(6x) √(3x2) = √(6x)(3x2)
Follow the normal rules to multiply the expression under the square root,
√(18x3)
Now put this expression in simple radical form, if required,
which means factoring out all the "perfect squares" from under the
square root.
Factor 18 into 92 because 9 is a perfect square (33)
Factor x3 into x2 x because x2 is a perfect square
√18x3 = √(9x2 2x) = √9x2 √2x
√9x2 = 3x (because 33=9, xx = x2, so 3x 3x = 9x2)
√9x2 √2x = 3x √2x, which can also be written
3x√2x