Please help me, I'm stuck on this question.

Step-by-step explanation:
AC=AB+BC
AC=x+4+3x‐8
COMBINE LIKE TERMS
AC=x+3x
AC =4x+4‐8
=4x+‐4
=0
Since the triangle is isosceles, then (x + 4) must be equal (3x - 8)
Solving the (x + 4) = (3x - 8), we'll get:
(x + 4) = (3x - 8)
3x - x = 8 + 4
2x = 12
x = 6
So, the length of AC is 6 units