Only for OTA 103997, 103300, 103642, 103866, 103139,or 103940 only

Only above OTAs-All others will be called in.
Stuck on this one-please explain-solved using ICE table.
0.5 HOAC
HOAC + H2O arrow both ways H3O^+ + OAC^-
Ka=[H3O^+] [OAC^-] / [HOAC]Why is product on bottom-always??
INItial 0.5,0,0
Change -x,+x,+x
Equil 0.5-x,+x,+x

Ka=1.8x10^-5 = (x)(x) / 0.5 -(x) = x2 /0.5 = 1.8 x 10^5
Lost now-please explain where this is coming from-how?:
x^2 = 9x10^-6
x= 3x10^-3
x=[H3O^+] = 3.0x10^-3 pH = 2.52