2a+9=6a-15

2a + 9  =  6a - 15

Get all the variable on one side; since 6a is bigger than 2a, subtract 2a from both sides:

2a - 2a + 9  =  6a - 2a - 15

     9  =  4a - 15

Add 15 to both sides:

     9 + 15  =  4a - 15 + 15

     24  =  4a

Divide both sides by 4:

     24 / 4  =  4a / 4

      6  =  a

11a=9a -> 11/9=0.8181818181818182=a