\(\overrightarrow{ a }=\frac{ q }{ m }(\overrightarrow{ v } \times \overrightarrow{ B })\)
So, \(\overrightarrow{ a }\,and\,\overrightarrow{ B }\) are \(\perp\) to each other
Hence, \(\vec{a} \cdot \vec{B}=0\)
\((\alpha \hat{i}-4 \hat{j}) \cdot(2 \hat{i}+3 \hat{j})=0\)
\(\alpha(2)+(-4)(3)=0\)
\(\alpha=\frac{12}{2} \Rightarrow \alpha=6\)