First, write your equation as two first order equations; xdot = v, say, and vdot = -9xdot - 0.1x^3
Then define two functions fx = @(v) v; and fv = @(x,v) -9*v-0.1*x^3;
Now you need to define two sets of k1 to k4: k1x = fx(v), k1v = fv(x,v), etc.
Then you will have X(n+1) = X(n) + 1/6*(k1x + 2*k2x + 2*k3x + k4x) and V(n+1) = V(n) + 1/6*(k1v + 2*k2v + 2*k3v + k4v)
Put the whole lot in a loop representing the times for which you want the values.
If the above doesn't make any sense to you then you need to do some introductory Matlab training.