Question

Y=3x

Answer 1

It's simple jus plug in 3x into Y and u will have 2x+3x=15 
                                                                              5x=15
                                                                               the answer(x=3)

Answer 2

So you subtitute y=3x into
2x+y=15
so
2x+(3x)=15
add like terms
5x=15
divide both sides by 5
x=3

subtitute x=3 into first equation (y=3x)
y=3(3)
y=9