I'm guessing you mean `theta = pi / 4` , otherwise this equation is already in rectangular coordinates.
To go from polar to rectangular, you want to use the fact that:
`"tan" theta = (y)/(x) `
So: `theta = pi / 4` means that ` "tan" theta = "tan" pi/4 = 1`
So: `(y)/(x)=1`, or `y=x`
You can think about this as the line whose angle with the x-axis is `pi/4` (that is the polar coordinate way to think about it)
Or you can think about this as the line with slope 1 and y-intercept 0 (that is the rectangular coordinate way)