Question 849778
Assign variables to everything and then use the description to refine what those variables are.

p = perimeter and p=51.

x, y, z are the sides of the triangle.
x = shortest
y = medium
z = longest
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z=2x.
y=x+3.
Notice how y and z both are in terms of x.  What then have you for x, y, z?
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x = shortest side
x+3 = medium side
2x = longest side


Meaning of perimeter means you have (x)+(x+3)+(2x)=p