The basis of each lateral triangle is one-half the difference between the basis of the trapezium:
triangle basis = (1/2)*(21 - 11) = 10/2 = 5
Now, using the Pythagorean Theorem, we have:
x² + 5² = 13²
x² + 25 = 169
To solve this equation, we can apply the same algebraic operations on both sides of the equation:
x² + 25 - 25 = 169 - 25
x² = 144
√x² = √144
x = 12
Therefore, option c is correct.