What is the value of b

 Yes, it is a right triangle

But only if you rounded everything to the nearest whole number.

Otherwise, it's a bit off (but fairly close).

Explanation:

If that was a right triangle, then a^2+b^2 = c^2 would be the case (pythagorean theorem). The a,b,c refers to the sides of the triangle. The 'c' is always the longest side, where 'a' and 'b' can be in any order you want.

We have

Which leads to

a^2+b^2 = c^2

(13.5)^2+(17)^2 = (21.7)^2

182.25 + 289 = 470.89

471.25 = 470.89

We don't get the same thing on both sides, so we don't have a right triangle.

However, your teacher mentions to round the results to the nearest whole number. The 471.25 on the left side becomes 471, while the 470.89 becomes 471.

So while the equation 471.25 = 470.89 is definitely false, both sides are close enough that they round to 471 = 471 which is true.

In other words, this isn't a right triangle but it's close enough to one. Based on this rounding criteria, Brian is correct.