In order to find a reference angle, we need to find the smallest possible angle formed between the x-axis and the terminal line of the given angle, going either clockwise or counterclockwise.
Since the given angle is 129°, and 90<129<180, it will look something like this:
As we can see, the reference angle will be
[tex]180-129=51[/tex]
so it will be 51°.
