Rearrange the equation to isolate h: a= 1/2bh

First we will multiply both sides by 2 to eliminate 1/2 from h.

2 x A = 1/2bh x 2
2A = bh

Now we divide both side by b to eliminate b from h.

2A / b = bh/b
2A/b = h

Idon’t know how to do that

I'm a bit puzzled here because they did not provide you with any measures of the other angles, so i am unable to provide a theorem or postulate to support my answer, but i'd say 90 degrees because it looks closer to a right angle than anything.