Answer
The speed of the blob of clay should be 300 m/s
Work Step by Step
In order to stop the car, the magnitude of the blob of clay's momentum $p_b$ should be equal to the magnitude of the car's momentum $p_c$. We can equate the magnitudes of the momenta to find the required speed $v$ of the blob of clay.
$p_b = p_c$
$(10~kg)~v = (1500~kg)(2.0~m/s)$
$v = 300~m/s$
The speed of the blob of clay should be 300 m/s.
