Answer
$\dfrac{1}{12}$
Work Step by Step
The fractions are not similar since they have different denominators. Make them similar fractions using their LCD (least common denominator).
To find the LCD, factor each denominator completely to have:
$\\28=7(4)
\\21=7(3)$
Thus, the LCD is $7(4)(3)=84$. Convert each fraction using the LCD as denominator to have:
$=\dfrac{5(3)}{28(3)}-\dfrac{2(4)}{21(4)}
\\\\=\dfrac{15}{84}-\dfrac{8}{84}
\\\\=\dfrac{7}{84}
\\\\=\dfrac{7}{7(12)}
\\\\=\dfrac{1}{12}$
