Answer: The rabbit is hopping at 3 feet/ sec in speed 
Step-by-step explanation:
Step 1
Let the The number of feet of the rabbit be expressed as N
 AND  time rabbit spends hopping can be expressed  as  t
From the question,  The number of feet varies directly with the time rabbit spends hopping can be expressed  as 
 N α t
Let us introduce a constant of proportionality ,S which represents  speed in this case, we now have that
 N = St
 STEP 2 -- SOLVING 
If the rabbit hop 6 feet in 2 seconds, Then speed =
N = St
6=  S X2
 S = 6/ 2 
S = 3
also 
If the rabbit hop 12 feet in 4 seconds, Then speed =
N = St
12=  S X 4
 S = 12/ 4 
S = 3
 Also recall
 that Speed = Distance / time
 Pluging the values in the first case,
 speed = 6 feet/ 2 secconds = 3 feet/ seconds
 in the second case 
 Speed= 12 feet/ 4 second s= 3 feet/ secs.