An investor wants to form a two asset portfolio consisting of treasury bills with a return of 2.5% and a risky portfolio with an expected return of 15.2% and a standard deviation of 16%. The investor wants the expected return of the two asset portfolio to be 13%.

a) What is the standard deviation of the two asset portfolio?

I know the answer is 13.23% but I don't know how to solve it(we cant use excel)