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When refering to the 20^o angle, the 10 cm side is the adjacent and y is the hypotenuse.

Since  cos = adj/hyp        cos(20^o)  =  10/y

Multiplying by y:          y · cos(20^o)  =  10

Dividing:                                       y  =  10 / cos(20^o)

 

Now, it's up to you and your calculator ...