I assume the diagram looks like this.Scalane triangle
Bottom right side is (x-15)deg
On the left corner there is an supplementary extension of 150°
so you subtract from180 and that gives you 30°.
It says: solve for x which is the top of the triangleCode:* / \ / x \ / \ / \ / \ / \ 150° / 30° x-15 \ * - - - - - - - - - - - *
You're expected to know that the three angles of a triangle add up to 180°.
So you have: .