#### Answer

(a) The acceleration is $2.4~m/s^2$
(b) The acceleration is $0.6~m/s^2$

#### Work Step by Step

Let $F$ be the force provided by one rubber band. With the information in the question, we can set up a force equation.
$2F = Ma$
$F = \frac{M~(1.2~m/s^2)}{2} = M~(0.60~m/s^2)$
(a) $4F = 4\times M~(0.60~m/s^2) = M~(2.4~m/s^2)$
The acceleration is $2.4~m/s^2$
(b) $2F = 2\times M~(0.60~m/s^2) = (2M)(0.60~m/s^2)$
The acceleration is $0.60~m/s^2$