This is very easy!! I have attached a picture I have drawn if you are a visual learner... well, everything else like computations is here.
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This is the PYTHAGOREAN THEOREM!!
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If you don't know, it says that the lengths of the legs squared added together all equal the hypotenuse squared in a right triangle!!
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And if you want to know why the "triangle" has a right angle... well, are towers slanted? I mean, other than the Leaning Tower of Pizza. I mean Pisa.
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So... we plug it in!!
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Formula: a^2 + b^2 = c^2.
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Plug it in: 11^2+ 61^2 = c^2
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c^2 = 3842.
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So now what?
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SQUARE ROOT OF EVERYTHING!!
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c = √3842.
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So? c = approximately 61.984.
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Hope I helped!!