[tex] \frac{2}{9} [/tex] - [tex] \frac{7}{12} [/tex]
Make the two fractions have common denominators. 9 and 12 both fit into 36, so multiply the top and bottom of [tex] \frac{2}{9} [/tex] by 4 and [tex] \frac{7}{12} [/tex] by 3.
[tex] \frac{2}{9} [/tex] ⇒ [tex] \frac{8}{36} [/tex]
[tex] \frac{7}{12} [/tex] ⇒ [tex] \frac{21}{36} [/tex]
Now that both fractions have common denominators, you can subtract them easily. Subtract the numerators (8 - 21) and keep the denominators the same.
[tex] \frac{8}{36} [/tex] - [tex] \frac{21}{36} [/tex] = [tex] \frac{-13}{36} [/tex]
a = -13
b = 36
