A15 ml sodium hydroxide solution of unknown concentration is titrated with 34.5ml of 1.2m hcl solution. what was the concentration of the sodium hydroxide solution?

NaOH + HCl = NaCl + H20
From the reaction, it can be seen that KOH and HCl have the following amount of substance relationship:
n(NaOH):n(HCl)=1:1
From the relationship we can determinate that NaOH and HCl have the same number of moles:
n(NaOH)=n(HCl)=c*V=1.2M*0.0345L= 0.00414 mole
Now, when moles of NaOH is known, concentration can be calculated: 
c(NaOH)=n/V= 0.00414mole/0.015L= 0.276 M

Your question is not clear but if you can explain yourself i would be glad to .  just trying to : ))