The length of side AC is 6.
In a triangle where angle A is congruent to angle C, it implies that sides AB and BC are congruent. Therefore, we have:
AB = BC
Given that AB = x + 4 and BC = 3x - 8, we can set up an equation:
x + 4 = 3x - 8
Now, solve for x:
2x = 12
x = 6
Now that we have the value of x, we can find the length of side AC:
AC = x
AC = 6
So, the length of side AC is 6.