Answer: The length AC equals 10.25 cm (approximately)
Step-by-step explanation: Please refer to the diagram attached for further details.
The triangle has been constructed according to the dimensions provided with the top B going down to the "little square inside the triangle" which is C. This clearly identifies it as a right angled triangle. The point B meets with point C at the right angle and A is the sharp tip of the triangle, which makes point A the third angle. Hence we now have a right angled triangle with side AB measuring 11 cm, side BC measuring 4 cm and side AC is unknown.
We can now apply the Pythagoras' theorem which is stated as follows;
AC² = BC² + AB²
Where AC is the hypotenuse (longest side) and BC and AB are the other two sides. We can now substitute for the appropriate sides in this question as all the sides have been labelled differently. Hence,
BA² = BC² + CA²
11² = 4² + CA²
121 = 16 + CA²
Subtract 16 from both sides of the equation
105 = CA²
Add the square root sign to both sides of the equation
√105 = √CA²
10.2469 = CA
CA ≈ 10.25 cm
The third side AC (or CA) measures approximately 10.25 cm