John should use his protractor to mark off a 70° angle whose vertex is one end of his 5 cm segment. If John is unsure how to do that, he can look up videos on how to use a protractor.
John could also construct a segment 13.74 cm long at the end of his segment that is perpendicular to the one he has. The angle formed between the other end of his 5 cm segment and the end of the 13.74 cm segment will be 70°.
Another choice John has is to draw an arc from each end of his 5 cm segment that has a radius of 7.3 cm. The point where these intersect over the middle of his segment will form an angle of 70° when a segment is drawn from there to the end of his 5 cm segment.
