\(F=\frac{v d m}{d t}-m g \ldots(i)\)
where \(m\) is mass of the rocket after 1 minute
\(\text { So }\)
\(m=[5700-15(60)]\)
\(=4800 \,kg\)
in \((i)\)
\(F=\left(12 \times 10^3\right)(15)-(4800) g\)
\(=(12000)(15)-48000\)
\(a=\frac{F}{m}=\frac{12000(15)-48000}{4800}\)
\(=27.5 \,m / s ^2\)