From Förberedande kurs i matematik 1

Subtraction works in the same way as addition. First, we multiply the top and bottom of each fraction by the other fraction's denominator so that they have a common denominator,

\displaystyle \frac{2\cdot 5}{7\cdot 5}-\frac{1\cdot 7}{5\cdot 7}=\frac{10}{35}-\frac{7}{35}\,.

Then the numerators can be subtracted,

\displaystyle \frac{10}{35}-\frac{7}{35}=\frac{10-7}{35}=\frac{3}{35}\,.