How do I find I? My bond is 5%, 6 years, semi-annual with a 6% market value? The solution says my n=12 that's right because you pay twice a year 2*6=12 but it says the I is 3% how did they get that?

i refers not to the coupon rate of the bond, which is fixed at issue (2 ½ % every six months, in this case), but rather to the effective yield an investor would earn by purchasing the bond at the current market price.

In this case we're told that the market has priced the bond to some amount such that a current buyer would earn a 6% yield from all the remaining cash flows, if he held it to maturity. The 6% is stated as an annual number, so i = 3% every six months.