Answer
The two lines/equations $coincide$ (overlap). Infinite solutions.
Work Step by Step
We use the addition method:
We add the 2 equations:
$3x-5y=7$
$9x-15y=21$
Multiply both sides of the first equation by $-3$ to cancel $x$.
$-9x+15y=-21$
$\underline{+9x-15y=+21}$
$0=0$
This is a true statement, which means that we have infinite solutions.
So, the two equations coincide (overlap).
