I believe the answer would be 9
Answer: Choice D) 
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Work Shown:

Explanation:
First I subtracted y/b from both sides. Then I multiplied both sides by 'a' to fully isolate x. You can optionally distribute the 'a' through to each term inside the parenthesis, but your teacher has chosen not to do this.
1_ 210+90= 300
2_ it's prime:
[1,19]
good luck
This question may be trick question xD haha.
Anyways, a right triangle is a triangle with a right angle (90 degrees). In your picture, that triangle is a right triangle, but the sides could be a bit slanted. Please take a picture from top view, so I can see it properly.
The triangle's length does not matter in this case. As long as it has a right angle, it's a right triangle.