Hello from MrBillDoesMath! 
 
Answer: 
23
 
Discussion: 
In ΔABC side AC is 11, but as the triangle is isosceles the other leg also has length 11. Let the third, unknown side, be called x. Then
11 + 11 + x = Perimeter = 45.
11 + 11 + x = 45 => as 11 + 11 = 22
22 + x = 45 => subtract 22 from both sides
x = 45 -22 = 23
 
 
Thank you, 
 
MrB