√3x·√51x

Break the 51 down to 3·17

√3x·√3·17x

Multiply under the radicals:

√3·3·17x²

√3²·17x²

Take out 3² as just a 3 in front of the radical.
Take out x² as just as |x| in front of the radical.
We must put the x between absolute value bars because the 
original problem is the product of two radical square roots,
which are never negative, therefore the answer cannot be negative.
But x is a variable so it might be negative. So to assure that the
answer is not negative we must put absolute value bars around
x.

Answer:

3|x|√17

Edwin