Answer:
P=30cm
Step-by-step explanation:
So the formula there is to find the Perimeter of the rectangle, so we are going to use it!
Using Substitution:

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The equation would be ' y = 1/2x'
The photo of my working will be attached.
Sometimes it helps to draw it out...
(17)
C_______M_________Q
|___________________|
21
therefore, CM + MQ = CQ
17 + MQ = 21
MQ = 21 - 17
MQ = 4 <===