Let's start by simplifying the multiplication:
24a - 6a + 15 = 51
Now let's simplify by doing the subtraction on the left side:
18a + 15 = 51
Now we'll simplify the addition by subtracting 15 from both sides:
18a = 36
Now we simplify the multiplication by dividing both sides by 18, and we have our final answer:
a = 2