$56)x+y+z=11$ where $x\geq0,y\geq0,z\geq0$
using star and bars
number of solutions$=\binom{11+3-1}{11}=\binom{13}{11}=\binom{13}{2}=78$
see this it is useful to read https://brilliant.org/wiki/integer-equations-star-and-bars/
$57)$ Referential integrity constraints works on the concept of the foreign key.