Question

Solve the system of equations.
2x+3y=1
y=3x+15
(?.?)

Answer 1

the way they are setting the system up they are offering substitution.

you have a system
\(\large\color{blue}{ \displaystyle 2x+3y=1}\)
\(\large\color{blue}{ \displaystyle y=3x+15}\)

you know that \(\large\color{blue}{ \displaystyle y}\) is the same thing as \(\large\color{blue}{ \displaystyle 3x+15}\) (based on the second equation).
So knowing this, you can replace \(\large\color{blue}{ \displaystyle y}\) in the first equation, with \(\large\color{blue}{ \displaystyle 3x+15}\).
(this replacement is called substitution.)

so lets do this "substitution".

\(\large\color{blue}{ \displaystyle 2x+3y=1}\)
\(\large\color{blue}{ \displaystyle 2x+3\color{red}{(3x+15)}=1}\)

Answer 2

Now, you can solve for x.