How do you evaluate 3ab

Lets say a=2 and b=5 you now plug in you numbers to get 3x2x5 we know 3x2=6 so now take 6 and multiply it by 5 so 6x5=30 you can also set this equal to 0 3ab=0 then divide by either a or b ill show you both if you divide by a you are left with 3b=a if you divide by b than you get 3a=b