2r-20 is the answer combining the like terms and solving.
Symmetric property is the correct term
You just find out what the missing variable is by subtracting 88 from 107 on the first one. You do the same for the last too as-well, but just insert the numbers that they say.
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Each of the triangles are equal in base length, height, and area<span>. Remember the formula for the </span>area<span> of a triangle. The </span>area<span> of any triangle is 1/2 times the length of the base (which, in the </span>polygon<span>, is the length of a side) multiplied by the height (which is the same as the apothem in </span>regular polygon<span>).
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