So like tangent theorem to solve this ?

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.

1st Method- Pythagoras:

16² = x² + 8²  → 256 = x² + 64 → x² 256 - 64 →x² = 192 and x =√192 = 13.85

2nd Method- Trigo

sin Ф = 8/16 = 1/2 and sin⁻¹ (1/2) = 30°

now tan 30° = 8/x
 0.57735 = 8/x and x = 8/(0.57735) = 13.85