what is 15a-54=_9

hi again GoldenLeaf,

Your logic and your answer are botf good.

however I don not like this line

15a(-54+54) = (9+54)

this implies 15a * (-54+54) = 15a * 0 = 0

and that is definitely not what you meant.

just write it like this - brackets usually help but in this case that make the working wrong.

15a-54+54 = 9+54

You may both teach and learn a lot here GoldenLeaf.  This is how I have always wanted the forum to be used.  

First, combine like terms. This means adding 54 to both sides

15a(-54+54) = (9+54)

15a = 63

Then, to separate 'a', divide by 15 on both sides

15a/15 = 63/15

a = 4.2

Unless the underscore before the 9 means something I am unaware of, then 4.2 is equivalent to 'a'