That would be the inverse property of multiplication. What the inverse property says is that, for every real number [tex] a [/tex], there is some special number [tex] a^{-1} [/tex] out there called the multiplicative inverse, which has the property that
[tex] a\cdot a^{-1}=1 [/tex]
In this case, [tex] a=\frac{9}{4} [/tex] and its inverse [tex] a^{-1}=\frac{4}{9} [/tex]
