Let's start with the given equation:
[tex]\[
\frac{4a}{b} = 12
\][/tex]
We aim to isolate [tex]\( a \)[/tex] on one side of the equation.
First, let's eliminate the fraction by multiplying both sides of the equation by [tex]\( b \)[/tex]:
[tex]\[
4a = 12b
\][/tex]
Next, we need to solve for [tex]\( a \)[/tex]. To do this, we'll divide both sides of the equation by 4:
[tex]\[
a = \frac{12b}{4}
\][/tex]
Simplifying the right-hand side, we get:
[tex]\[
a = 3b
\][/tex]
Therefore, the expression equivalent to [tex]\( a \)[/tex] is:
[tex]\[
a = 3b
\][/tex]
So, the correct answer is:
[tex]\[
\boxed{3b}
\][/tex]
