2 A straight \(\operatorname { rod } A B\) has length \(a\). The rod has variable density, and at a distance \(x\) from \(A\) its mass per unit length is \(k \mathrm { e } ^ { - \frac { x } { a } }\), where \(k\) is a constant. Find, in an exact form, the distance of the centre of mass of the rod from \(A\).