I would use ABCD parameters, concatenating the ABCD parameters for the 50 ohm source resistor with the abcd parameters for the xmsn line. This method can handle any kind of oddball mismatches, such as in this case the 300 ohm xmsn line to the 50 ohm source, and any desired length of line (here λ/4) and far-end termination impedance (here ∞).
I'm sure that's overkill here & that there's an esier way, as skeptic2 is suggesting. Just pointing out that abcd parameters can handle the most general cases.