$\frac{ b \times \left( a-b \right)-a \times \left( b-a \right) }{ ab }$
Distribute $b$ through the parentheses$\frac{ ab-{b}^{2}-a \times \left( b-a \right) }{ ab }$
Distribute $-a$ through the parentheses$\frac{ ab-{b}^{2}-ab+{a}^{2} }{ ab }$
Since two opposites add up to $0$, remove them from the expression$\frac{ -{b}^{2}+{a}^{2} }{ ab }$