Find 'i' over 'u'

<=>  -9x + 7i > -3 (3x + 7u)
<=>  -9x + 7i > -9x - 21u
<=>           7i > -21u               | (: 7)
<=>             i > -3u
<=>             i < 3u

* I was surprised a dude ask me for calculating this algebra.

YOU, good luck for the love math :)