TUYMAADA 2015 S6

Let $0 \leq b \leq c \leq d \leq a$ and $a>14$ are integers. Prove, that there is such natural $n$ that can not be represented as $$n=x(ax+b)+y(ay+c)+z(az+d)$$ where $x,y,z$ are some integers.

K. Kohas