Plx help me to solve the question of normal distrubution

It is the first time to ask question here. I have a question in my assignment cannot solve, plx help me to solve that question, plx, plx, i reli need the answer, otherwise, i will very sad ga

Let x~N(u, a^2). Find k such that P(u-ka < x < u+2ka) =0.8.
where the a is the sigma, i cannot type it in IE, so use a instead.

thx!!! ^^