Answer
$\frac{x-21}{7x}$
Work Step by Step
$\frac{1}{7}-\frac{3}{x}$ $\longrightarrow$ convert to like fractions
=$\frac{1\times x}{7\times x}-\frac{3\times7}{x\times7}$ $\longrightarrow$simplify
=$\frac{x}{7x}-\frac{21}{7x}$ $\longrightarrow$ subtract numerators
=$\frac{x-21}{7x}$
