The two basic laws used are
1 ] conservation of mass
2] conservation of charge 13Al27+2He4→15P30+X
as we can see that no. of protons on both sides are balanced already. 13+2=15
but masses are not balanced,
reactant side has more mass by one unit than P30
therefore X carries 1 unit mass. X is a particle with 0 charge and 1 unit mass.
Hence, it is a neutron.