It's 192, I'm pretty sure. I hope that you get this right! :^)
You need to implement the process of "flip-flop" and multiply which is where you flip the second fraction and multiply.

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Now you multiply across. 2×15=30 and 3×11=33
So now it is

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To reduce to the lowest terms, find the GCF common factor
The GCF is 3
Divide both the numerator and denominator by 3

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Answer:
Hypotenuse² = perpendicular² + base²
H² = 20²+21²
H²= 400+441 = 841
H = √841
H = 29 cm