[tex]\Large\boxed{\begin{array}{l}\sf x+2y=0\implies x=-2y\\\sf a=-2\cdot1=-2\\\sf 2=-2b\\\sf b=-\dfrac{2}{2}\\\sf b=-1\\\sf\dfrac{5a}{2b}=\dfrac{5\cdot(-\diagup\!\!\!2)}{\diagup\!\!\!2\cdot(-1)}=5\\\huge\boxed{\boxed{\boxed{\boxed{\sf\maltese~alternativa~a}}}}\end{array}}[/tex]