Block $B$ starts from rest and accelerates as ${\mathrm{a}}_{\mathrm{B}}= 12t \mathrm{m} {\mathrm{s}}^{-2}$. Simultaneously another end of the string is pulled with constant acceleration ${\mathrm{a}}_0$. If at $t=3 \mathrm{s}$, block $A$ comes to rest then the value of ${\mathrm{a}}_0$ (in $\mathrm{m} {\mathrm{s}}^{-2}$) is equal to