Question

M is between points C and Q. CM=17 and CQ=21. What is MQ?

Answer 1

Sometimes it helps to draw it out...

     (17)
C_______M_________Q
|___________________|
             21

therefore, CM + MQ = CQ
17 + MQ = 21
MQ = 21 - 17
MQ = 4 <===

Answer 2

we know that

If M is between points C and Q

then

$CQ=CM+MQ$

$CM=17\ units\\ CQ=21\ units\\MQ=?$

Solve for MQ

$MQ=CQ-CM$

Substitute the values in the formula above

$MQ=21-17$

$MQ=4\ units$

therefore

the answer is

$MQ=4\ units$