What is the constant difference between the consecutive terms?

$3 - 1 = 2$  
$5 - 3 = 2$  
$7 - 5 = 2$  
The constant difference is $\boxed{\text{2}}$.

$18 - 21 = -3$  
$15 - 18 = -3$  
$12 - 15 = -3$  
The constant difference is $\boxed{\text{-3}}$.

$170 - 175 = -5$  
$165 - 170 = -5$  
$160 - 165 = -5$  
The constant difference is $\boxed{\text{-5}}$.

$342 - 336 = 6$  
$348 - 342 = 6$  
$354 - 348 = 6$  
The constant difference is $\boxed{\text{6}}$.

$4 - 3.5 = 0.5$  
$4.5 - 4 = 0.5$  
$5 - 4.5 = 0.5$  
The constant difference is $\boxed{\text{0.5}}$.

$10.6 - 10.8 = -0.2$  
$10.4 - 10.6 = -0.2$  
$10.2 - 10.4 = -0.2$  
The constant difference is $\boxed{\text{-0.2}}$.