It appears that you subtracted the numerators and subtracted the denominators. Instead, you need to find the least common denominator, which is $\displaystyle{8}{a}\cdot{7}{b}={56}{a}{b}\usepackage{cmbright}\Huge\$ . Then rewrite both fractions so that they have this denominator. Finally, subtract only the numerators while keeping the denominator the same. The correct answer is: $\displaystyle\frac{{{21}{b}-{16}{a}}}{{{56}{a}{b}}}\usepackage{cmbright}\Huge\$