I'll do problem 12 to get you started
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Part (a)
The ratio 3:6:7 means the three sides are 3x, 6x and 7x centimeters long. The x is some positive real number.
The longest side is 7x, which is also 16.1, so 7x = 16.1
Divide both sides by 7 to isolate x
7x = 16.1
7x/7 = 16.1/7
x = 2.3
Now we can calculate the other two sides
3x = 3*2.3 = 6.9
6x = 6*2.3 = 13.8
The other two sides are 6.9 cm and 13.8 cm
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Part (b)
Add up the three sides to get
6.9 + 13.8 + 16.1 = 36.8
The perimeter is 36.8 cm.
