ANSWER
![x = 3](https://img.qammunity.org/2020/formulas/mathematics/high-school/gn0ewdpib9ooyfthsjwp8oi4900vxj88n6.png)
Step-by-step explanation
If the equation is parallel to the y-axis, then its slope is undefined.
The equation of a line that is parallel to the y-axis and passes through
![(x_1,y_1)](https://img.qammunity.org/2020/formulas/mathematics/high-school/19r3d7du1thfjdcyjucmq84asmb0fngoya.png)
is given by:
![x = x_1](https://img.qammunity.org/2020/formulas/mathematics/high-school/saqfwvyk3eopc3tgyiv93x70av09rkq7yp.png)
The given line is parallel to the y-axis and passes through (3,7).
It's equation is
![x = 3](https://img.qammunity.org/2020/formulas/mathematics/high-school/gn0ewdpib9ooyfthsjwp8oi4900vxj88n6.png)