[yazz]

ksp=3.7*10*-15 for AlOH3. A precipitate just began to form when 50.0ml of 2.0*10*-6M of AlOH3 was mixed with 50ml of NaOH solution. What was the concentration of the NaOH solution?

AlOH3= Al + 3OH
3.7*10*-15/2.0*10*-6=3OH
OH=6.1*10*-10 M

i don't know whether this is the way to do it or not. please help

[Yggdrasil]

You're on the right track, but your answer is incorrect.  First, what is the concentration of aluminum ions in your final solution (hint: it's not 2x10^-6M).  Second, it would be beneficial to write out the equation for K_sp in terms of [Al³⁺] and [OH^-].  This may help you find the correct [OH^-].