Answer:
x + 2x = 9
General Formulas and Concepts:
Algebra I
Step-by-step explanation:
Since we are using substitution, we must replace one variable in 1 equation as the 2nd equation.
We are given:
y = 2x
x + y = 9
Here we see that y equals 2x. We can go ahead and replace in the 2nd equation y.
x + 2x = 9
And we would solve from there.