what must be subtracted from 2 1/7 to get 17/21?

Taking the given fractions as $2\frac{1}{7} \mathrm{and} \frac{17}{21}$
Suppose x be the required number to be subtracted from $2\frac{1}{7}$.
So we have;
$$\begin{gathered} 2\frac{1}{7}-x = \frac{17}{21} \\ \Rightarrow \frac{15}{7}-x = \frac{17}{21} \\ \Rightarrow x = \frac{15}{7}-\frac{17}{21} \\ \Rightarrow x = \frac{45-17}{21} = \frac{28}{21} = \frac{4}{3} = 1\frac{1}{3} \\ \mathrm{Therefore} \mathrm{the} \mathrm{fraction} \mathrm{to} \mathrm{be} \mathrm{subtracted} \mathrm{is} 1\frac{1}{3}. \end{gathered}$$