To calculate the hypotenuse of a right angled triangle as shown in the diagram, we can apply the Pythagoras theorem which states as follows;
AC^2 = AB^2 + BC^2
Where AC is the hypotenuse (the longest side) 28 units, AB is one of the other two sides, 23 units and BC is the third side.
We can now re-write the formula as follows;
28^2 = 23^2 + BC^2
784 = 529 = BC^2
Subtract 529 from both sides of the equation
255 = BC^2
Add the square root sign to both sides of the equation and you have
BC = 15.9687
BC is approximately 16 units