3(a+4)-9=

First, use the distributive property:  3(a + 4)  =  3(a) + 3(4)  =  3a + 12

3(a + 4) - 9  =  3a + 12 - 9  

Since 12 - 9  =  3:

3(a + 4) - 9  -  3a + 12 - 9  =  3a + 3