Recall that with constant acceleration,
[tex]x_f-x_i=\dfrac{v_i+v_f}2t[/tex]
where [tex]x_f-x_i[/tex] is the rocket's displacment, [tex]v_i[/tex] is its initial velocity, [tex]v_f[/tex] its final velocity, and [tex]t[/tex] is time. So
[tex]x_f-x_i=\dfrac{80\frac{\mathrm m}{\mathrm s}+152\,\frac{\mathrm m}{\mathrm s}}2(18\,\mathrm s)=2088\,\mathrm m[/tex]
or just 2100 m if taking significant digits into account.
