Step 1: Recall what isotonic means.
Two solutions are isotonic when they exert the same osmotic pressure at the same temperature. Since osmotic pressure is $\pi = CRT$, equal pressure at equal temperature means equal molar concentration.
Step 2: Write the equality of concentrations.
If both solutions have the same volume of solvent, equal molarity means equal number of moles of solute. So moles of urea equal moles of $X$. \[ \frac{w_{urea}}{M_{urea}} = \frac{w_X}{M_X} \]
Step 3: Note the molar mass of urea.
Urea is $NH_2CONH_2$ with molar mass $60\;g\;mol^{-1}$.
Step 4: Insert the given masses.
Here the urea mass is $6.0\;g$ and the mass of $X$ is $10\;g$. \[ \frac{6.0}{60} = \frac{10}{M_X} \]
Step 5: Simplify the left side.
\[ 0.1 = \frac{10}{M_X} \]
Step 6: Solve for the molar mass of $X$.
Rearranging gives $M_X = \dfrac{10}{0.1} = 100$. So the molar mass of the solute $X$ is
\[ \boxed{100\;g\;mol^{-1}} \]