1)split into two fractions
[tex] \frac{50a^{2} }{20a ^{3} b ^{3} } [/tex] - [tex] \frac{10ab }{20a ^{3} b ^{3} } [/tex]
2) cancel what you can on both sides
[tex] \frac{5}{2ab ^{3} } [/tex] - [tex] \frac{1}{2a ^{2}b ^{2} } [/tex]
3) Make the denominators the same by cross multiplying and then put together as 1 fraction again
[tex] \frac{5a-b}{2a ^{2}b ^{2} } [/tex]