Answer: 9 units
========================================================
Step-by-step explanation:
For now, focus on triangle ABD. This is a right triangle due to BD being an altitude.
Use the pythagorean theorem to find that...
(AD)^2 + (BD)^2 = (AB)^2
(1)^2 + (BD)^2 = (3)^2
1 + (BD)^2 = 9
(BD)^2 = 9-1
(BD)^2 = 8
Normally we would isolate BD itself, but I'll stop short and use that last line instead.
------------------
The triangles ABD and BCD are similar triangles (we can prove this using the AA similarity theorem).
Since we have similar triangles, we can form the proportion shown below
AD/BD = BD/CD
This cross multiplies to
AD*CD = (BD)^2
We know that AD = 1 and (BD)^2 = 8, so,
AD*CD = (BD)^2
1*CD = 8
CD = 8
This then means,
x = CD+DA
x = 8+1
x = 9
AC is 9 units long.