[itex]r = r_0 + 0.9t[/itex] is only valid if [itex]dr/dt[/itex] is constant.
Why did you assume that [itex]dr/dt[/itex] was constant? The question only tells you that [itex]dr/dt = 0.900\,\mathrm{cm}/\mathrm{s}[/itex] when [itex]r = 6.50\,\mathrm{cm}[/itex].