To solve this plug in 9 for b and solve for a like so...
3a + 9 = 54
9 must be combined with 54. To do this you must subtract 9 from both sides. This will cancel out 9 from the left and bring it over to the right
3a + (9 - 9) = 54 - 9
3a + 0 = 45
3a = 45
Now we must isolate the a. To do this you have to divide 3 to both sides. This will make 3 on the left side 1 and bring 3 over to the other side
3a/3 = 45/3
1a = 15
a = 15
Hope this helped!
~Just a girl in love with Shawn Mendes