need help

[untold stump]

can someone please explain how to do this?

[supple chasm]

Find the line D first by doing 10^2 -6^2

Gives you 64 root of that is 8 meaning line D is 8

Therefore line BC is 8 as well

Angle B is right angle so we can use Pythagorean theroem

8^2 + 9^2 = AC^2

= 145

Square root of 145 is 12.0415

Three sig figures == 12.0