A packing crate with mass $80.0 \mathrm{~kg}$ is at rest on a horizontal, frictionless surface. At $t=0$ a net horizontal force in the $+x$ -direction is applied to the crate. The force has a constant value of $80.0 \mathrm{~N}$ for $12.0 \mathrm{~s}$ and then decreases linearly with time so it becomes zero after an additional $6.00 \mathrm{~s}$. What is the final speed of the crate, $18.0 \mathrm{~s}$ after the force was first applied?