Multiplying an integer by a fraction can be treated as the following operation:
[tex] \frac{3x}{1} * \frac{1}{6x} = \frac{3x}{6x} [/tex]
The x's cancel out, since they're both on the numerator and denominator, so you'll be left with this fraction:
[tex] \frac{3}{6} [/tex]
The fraction can be further simplified by dividing both the numerator and denominator by 3:
[tex]\frac{3}{6} \div \frac{3}{3} = \frac{1}{2}[/tex]
The answer is 1/2.