To solve the equations:
[tex]\[
\begin{array}{l}
y = 2x \\
x + y = 9
\end{array}
\][/tex]
we can use the substitution method. Here’s how you do it step-by-step:
1. From the first equation, we already have [tex]\( y \)[/tex] expressed in terms of [tex]\( x \)[/tex]:
[tex]\[
y = 2x
\][/tex]
2. Next, we substitute [tex]\( y \)[/tex] into the second equation [tex]\( x + y = 9 \)[/tex]. This means replacing [tex]\( y \)[/tex] with [tex]\( 2x \)[/tex] in the second equation:
[tex]\[
x + 2x = 9
\][/tex]
So, the first step of substitution looks like this:
[tex]\[
x + 2x = 9
\][/tex]
Therefore, the correct option corresponding to this substitution step is:
d. [tex]\( x + 2x = 9 \)[/tex]
