For 3 kg mass: F = [tex]\lpha ma,a[/tex] = [tex]\frac{F}{m}[/tex] = [tex]\frac{12N}{3kg}[/tex] = 4[tex]\lpha m[/tex] / [tex]s^{2}[/tex]
For 6 kg mass: F = [tex]\lpha ma,a[/tex] = [tex]\frac{F}{m}[/tex] = [tex]\frac{12N}{6kg}[/tex] = 2[tex]\lpha m[/tex] / [tex]s^{2}[/tex]
Hope this helped, have a great day!~