I suppose both triangles are similar.
The ratio of 2 sides in a triangle is the same ratio of the corresponding 2 sides in the second similar triangle.
Which means,
AB / AC = LM / LN
now we can substitute the given values in
5 / (x + 5) = 10 / (3x + 3)
Use the cross method to solve this.
10(x+5) = 5(3x+3)
10x + 50 = 15x + 15
50 - 15 = 15x - 10x
35 = 5x
X = 7
Now we know the value of x is calculate the length of AC
AC = 7 + 5
Therfore the answer is 12