formula required ...
v is linear speed
r is radius
is angular speed in radians per unit time
first, a unit conversion ...
11 mph = (968/5) in/sec
968/5 = 14
= (484/35) radians/sec
angular speed of the rear wheel = angular speed of the rear sprocket
484/35 = (linear speed of the rear sprocket)/3
linear speed of the rear sprocket = (1452/35) in/sec = linear speed of the chain connecting the front and rear sprockets.
now you know the linear speed of the front sprocket. convert the angular speed of the front sprocket from rev/sec to rad/sec and calculate its radius.