Please Help me answer this question, I received my exam back but got this question wrong.

A 100 mg sample of radioactive actinium 227 decays according this equation:

When N is the number of milligrams present after t years . Find the half life of 227 Actinium to the nearest 0.1 of a year. [The half life is when half of the original amount is still present]

Is this right?:

I know I have to use logs, but I don't know how to finish because of the .

Please help!