use the pythagorean theorem
[tex] {a}^{2} + {b}^{2} = {c}^{2} [/tex]
input your values
[tex] {20}^{2} + {21}^{2} = {c}^{2} [/tex]
solve
[tex] {20}^{2} + {21}^{2} = 41 {}^{2} [/tex]
then square root 'c'
[tex] \sqrt{41} = 6.4 \: or \: \sqrt{41} [/tex]
these options are for if your teacher wants it in radical form as mine had
