.
a + b + c is the value of the right hand side at x = 1.
Hence, a + b + c is equal to the value of the left hand side at x= 1, i.e.
a + b + c = (3*1 + 5)^2 = 8^2 = 64.
Answer. a + b + c = 64.
The lesson to learn from this solution:
You do not need to open parentheses and manipulate with the coefficients.
There is much more simple and straightforward way,
and the problem simply wants to check if you know this way.
Good problem for an interview on a Math position, when the employer wants to check if you are/were trained in Math.