Question

50 points + brainliest for correct answer and explanation :)

Answer 1

The triangle ABC and the triangle BDC are similar, therefore:

$\dfrac{21+4}{x}=\dfrac{x}{4}\ \ \ |cross\ multiply\\\\x^2=25\cdot4\\\\x^2=100\to x=\sqrt{100}\\\\\boxed{x=10}$

Answer 2

The smallest and largest  (whole ) triangle are similar so 

x / 25 = 4 / x
x^2 = 100
x = 10  answer