Hint: Find the interior angle in a decagon.
Thought process: Using the formula, 180(n-2), 180(8)=1440-total degrees. We can divide by 10 to find the interior angle of each side, 1440/10=144.
And, if you see the line is cutting exactly in the middle, so we have to x angles. We also have 2 decagon angles, which sum to 144(2)=288.
Since there are 360 degrees in a quadrilateral, we yield with the equation of 2x+288=360, 2x=72, x=36 degrees.
I hope I didn't make any silly mistakes!