\(y=\sqrt{13}\left(\frac{2}{\sqrt{13}}cos3x+\frac{3}{\sqrt{13}}sin3x\right)-2\)
\(=\sqrt{13}sin\left(3x+a\right)-2\) với \(a\in\left(0;\pi\right)\) sao cho \(cosa=\frac{3}{\sqrt{13}}\)
\(\Rightarrow-\sqrt{13}-2\le y\le\sqrt{13}-2\)
\(\Rightarrow y=\left\{-5;-4;-3;-2;-1;0;1\right\}\) có 7 giá trị nguyên