Ta có: \(\dfrac{2a}{5}=\dfrac{3b}{2}\)
\(\Rightarrow4a=15b\)
\(\Rightarrow\dfrac{a}{15}=\dfrac{b}{4}\)
Đặt \(\dfrac{a}{15}=\dfrac{b}{4}=k\)
\(\Rightarrow\left[{}\begin{matrix}a=15k\\b=4k\end{matrix}\right.\)
Thay vào đề bài: \(15k+4k=38\)
\(\Rightarrow19k=38\)
\(\Rightarrow k=2\)
\(\Rightarrow\left[{}\begin{matrix}a=15.2=30\\b=4.2=8\end{matrix}\right.\)
Khi đó: \(3.30-2.8=74\)
Vậy \(3a-2b=74\).