Hey Maplesnowy!
Since a full circle is 360 degrees and we are missing one angle, we can write an equation to solve for that angle.
Also, a right angle is 90 degrees.
90 + 90 + 150 + x = 360
330 + x = 360
x = 30
The angle you want to solve for is 30 degrees.
I hope this helped,
Gavin.