To find the value of a in the equation 3a + b = 54 when b = 9, we can start by substituting 9 for b in the equation:
3a + 9 = 54
Next, we need to isolate 3a. We do this by subtracting 9 from both sides of the equation:
3a = 54 – 9
3a = 45
Now, we need to solve for a by dividing both sides by 3:
a = 45 / 3
a = 15
Therefore, the value of a is 15.