To find the value of the expression [tex]\( 21 - 2a \)[/tex] when [tex]\( a = 3 \)[/tex], follow these steps:
1. Substitute the given value of [tex]\( a \)[/tex] into the expression:
[tex]\[
21 - 2a
\][/tex]
When [tex]\( a = 3 \)[/tex], the expression becomes:
[tex]\[
21 - 2 \times 3
\][/tex]
2. Perform the multiplication first, according to the order of operations (PEMDAS/BODMAS):
[tex]\[
2 \times 3 = 6
\][/tex]
3. Substitute the result of the multiplication back into the expression:
[tex]\[
21 - 6
\][/tex]
4. Perform the subtraction:
[tex]\[
21 - 6 = 15
\][/tex]
Thus, the value of the expression [tex]\( 21 - 2a \)[/tex] when [tex]\( a = 3 \)[/tex] is [tex]\( 15 \)[/tex].
Therefore, the correct answer is:
D) 15
