You can use the Pythagorean Theorem since it's indicated in the problem that the triangle is right (has a 90 degree angle in it).
So what you can do is use: a^2 + b^2 = c^2
8^2 + x^2 = 16^2
64 + x^2 = 256
x^2 = 192
x = 8√3
I don't think you can use tangent in this problem since no other interior angles are given.