Answer:
12.449899598
Step-by-step explanation:
You would use pythagoras thereom.
C^2=A^2+B^2
The hypotenuse is C (which is the longest side of the triangle - 18).
18×18=13×13+B^2
324=169+B^2
324-169=B^2
155=B^2
Square root B to get B on it's own which would be 12.449899598.
