Answer: All real numbers.
Step-by-step explanation:
4(a+3) = 12 + 4a
4a + 12 = 12 + 4a
Any value of a will work in this equation, since it is a simple identity. 4a + 12 will always equal 4a + 12, regardless of a.
All real numbers, although I don't know why imaginary numbers wouldn't also work.
