#### Answer

right

#### Work Step by Step

In an acute triangle, the square of the length of the longest side is shorter than the squares of the lengths of the other two sides; therefore, $c^2 < a^2 + b^2$ or $a^2 + b^2 > c^2$.
In an obtuse triangle, the square of the length of the longest side is longer than the squares of the lengths of the other two sides; therefore, $c^2 > a^2 + b^2$, or $a^2 + b^2 < c^2$.
Finally, in a right triangle, the square of the length of the longest side is equal to the squares of the lengths of the other two sides; therefore, $c^2 = a^2 + b^2$, or $a^2 + b^2 = c^2$.
Let's find out what situation exists for the triangle with the given sides:
$15^2 + 36^2$ ? $39^2$
Evaluate the exponents:
$225+ 1296$ ? $1521$
Add to simplify:
$1521 = 1521$
$a^2 + b^2 = c^2$; therefore, this triangle is a right triangle.