The area of a triangle is .5(base)(height).  Here, area and height BC are given, so you need to solve for the unknown base AC:
 A = .5(AC)(BC)
 100 = .5(AC)(20)
 100 = 10AC
 AC = 10
 Using the Pythagorean Theorem, you can solve for y thus:
 y² = AC² + BC²
 y = square root of(102 + 202) = square root of 50 = 10x square root of 5, which is B

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