The possibilities are:

2 from 6 math

2 from 7 science

2 from 5 economics

You shall find the number of combinations for each one add them them together for they are disjoint.

The possibilities are:b)the book are to be on different subject

thanks

1 from 6 maths, 1 from 7 science

1 from 6 maths, 1 from 4 economics

1 from 7 science, 1 from 4 economics

The add them up,