If c is the longest side, then c must be equal to 10 in order to have a right triangle. This is due to the pythagorean theorem
a^2+b^2 = c^2
6^2+8^2 = c^2
36+64 = c^2
100 = c^2
c^2 = 100
sqrt(c^2) = sqrt(100) ... apply square root to both sides
c = sqrt(100)
c = 10
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So if we know that c = 10, then the side lengths 6,8,c form a right triangle.
