Answer:

Step-by-step explanation:
First off, your drawing is kinda inaccurate, because the
looks like a right angle, but as you drew it yourself it doesn't matter too much.
We know that the sum of the angles of a triangle will always equal
, so we have that
.
Combining like terms on the left side gives
.
Subtracting
from both sides gives
.
So,
and we're done!