The 3 inside angles of a triangle need to equal 180 degrees. Looking at the larger outside triangle the top angle is 30 and the lower right angle is 60.
30 + 60 = 90
The lower left angle needs to be 180 - 90 = 90 degrees.
That angle is split into two angles with one given as 70.
This means X needs to be 90 - 70 = 20 degrees.
x = 20
