Sure, let's solve the problem step-by-step:
1. We are given two equations:
[tex]\[
y = 2x
\][/tex]
[tex]\[
x + y = 9
\][/tex]
2. In the second equation, [tex]\( x + y = 9 \)[/tex], we need to substitute the value of [tex]\( y \)[/tex] from the first equation. Since [tex]\( y = 2x \)[/tex], we replace [tex]\( y \)[/tex] in the second equation with [tex]\( 2x \)[/tex]:
[tex]\[
x + 2x = 9
\][/tex]
So the first step in the substitution problem is:
d. [tex]\( x + 2x = 9 \)[/tex]