$\begin{array}{l}
\,\,\,\,\,\,\,The\,equation\,of\,trajectory\,is\\
\,\,\,\,\,\,\,\,y = x\tan \theta  – \frac{{g{x^2}}}{{2{u^2}{{\cos }^2}\theta }}\\
Where\,\theta \,is\,the\,angle\,of\,projection\,and\\
u\,is\,the\,velocity\,with\,which\,projectile\\
is\,projected.\\
For\,equal\,trajectories\,and\,for\,same\,angles\\
of\,projection,
\end{array}$

$\begin{array}{l}
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{g}{{{u^2}}} = {\rm{constant}}\\
As\,per\,question,\,\frac{{9.8}}{{{5^2}}} = \frac{{g'}}{{{3^2}}}\\
Where\,g'\,is\,acceleration\,due\,to\,gravity\,on\\
the\,planet.\\
\,\,\,\,\,\,\,\,\,\,\,\,\,g' = \frac{{9.8 \times 9}}{{25}} = 3.5\,m\,{s^{ – 2}}
\end{array}$