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Let’s assume T is between P and Q and we have to find the length of PQ in terms of x: 3x+6+6x-9=9x-3.
Now let’s assume T is the midpoint of PQ, so PT=TQ, that is, 3x+6=6x-9, so we can find x. Subtract 3x from each side of the equation:
6=3x-9, then add 9 to each side: 15=3x, 5=x, which is the same as x=5.
So now we can find PT=3x+6=15+6=21=TQ. Since T is the midpoint of PQ, PQ=21+21=42.
This should help.