2 answers

(click or scroll down)

## assuming that x is in degrees, then knowing that cos(x) = sin(90-x), we have

sin(2x+30) = sin(90-(x-15))

sin(2x+30) = sin(105-x)

so,

2x+30 = 105-x

3x = 75

x = 25

check:

sin(80) = cos(10)

yep.