You can cross multiply or multiply both sides by 26
[tex]\dfrac{a}{26}=\dfrac{24}{13}\\\\26\!\!\!\!\!\diagup^1\cdot\dfrac{a}{26\!\!\!\!\!\diagup_1}=26\!\!\!\!\!\diagup^2\cdot\dfrac{24}{13\!\!\!\!\!\diagup_1}\\\\1\cdot a=2\cdot24\\\\\boxed{a=48}[/tex]
