Hey mate, I took a quick look at your triangle.
I know ther eis some formula for these thing. But if you're like me, you probably don't know it and don't wana learn it.
So I'll tell you how I had do it.
I would split the upper angle (in this case B) and make a line from it to the botom of the triangle. This way I would have 2 right angled triangle.
With right angled triangle everything should be easier! With sin/cos and tan you should be able to work it ou!