To find the value of [tex]\( a \)[/tex] in the equation [tex]\( 3a + b = 54 \)[/tex] when [tex]\( b = 9 \)[/tex], we can follow these steps:
1. Substitute the value of [tex]\( b \)[/tex] into the equation:
[tex]\[
3a + 9 = 54
\][/tex]
2. To isolate [tex]\( 3a \)[/tex], we need to remove the constant term on the left-hand side. Subtract 9 from both sides of the equation:
[tex]\[
3a = 54 - 9
\][/tex]
3. Simplify the right-hand side:
[tex]\[
3a = 45
\][/tex]
4. To find the value of [tex]\( a \)[/tex], divide both sides of the equation by 3:
[tex]\[
a = \frac{45}{3}
\][/tex]
5. Simplify the division:
[tex]\[
a = 15
\][/tex]
Therefore, the value of [tex]\( a \)[/tex] is [tex]\( 15.0 \)[/tex], which corresponds to the answer choice:
[tex]\(\boxed{15}\)[/tex]
