"rotate the pedals 3x to make the rear tire rotate 360 degrees" means
rotate the pedals 1x to make the rear tire rotate 120 degrees [that is, (1/3)*360°]
So, what is (1/3) of the circumference of a circle with diameter 34"?
The definition of π (pi) is the ratio of the circumference of a circle to its diameter (π = C/d). So, C = πd.
(1/3)C = (1/3)(π)(34")
To check (or approximate):
Since Pi is a little more than 3 and the diameter is 34", then the bicycle will travel approximately (a little more than) 34" with one rotation of the pedals.