Answer:
x = 4√7
Step-by-step explanation:
You can use the pythagorean theorem which is a² + b² = c²
if you plug it in, it would be...
x² + 12² = 16²
x² + 144 = 256, subtract 144 to both sides
x² = 112, square root both sides to make x by itself.
x = 4√7
