ANSWER
The restrictions are

Step-by-step explanation
We were given the rational function,

The function is defined for all values of a, except

This has become a quadratic trinomial, so we need to split the middle term.
We do that by multiplying the coefficient of
which is 5 by the constant term which is 3. This gives us 15.
The factors of 15 that adds up to 16 are 1 and 15.
We use these factors to split the middle term.

We now factor to get,

We factor further to get,

This implies that,

This gives

These are the restrictions.