Question

Sin (x)= 0.78 what is x?

Answer 1

The angle whose sin is 0.78 is either 51.26 degrees or 128.74 degrees.
(rounded)

There's no simple way to calculate this.  You just have to look it up in a book
or punch it into your calculator.

Answer 2

Sin (x)=0.78

x=$sin^{-1} (0.78)$

x= 51.26°   or     180°- 51.26°= 128.74°