k = 100
u[x_] := (∑_(n = 1)^k)(Sin[(1/6)(n*π*
                  x)]*(-2/(n*π)*(Cos[(1/2)(n*π)] - Cos[(1/6)(π*n)])*
                Cos[(1/6)(n*π*t)]))
Table[
      Plot[u[x], {x, -1, 7}, 
        PlotStyle -> {RGBColor[1, 0.7, 0], Thickness[0.015]}, 
        PlotRange -> {{0, 6}, {-1.1, 1.1}}], {t, 0, 18, 0.1}]