Question 1107691
They are confusing you with the statement "the quadrilateral". You should only be concerned with the right triangle with sides 15 and x, and hypotenuse 25.
Apply the Pythagoras theorem:
a^2 + b^2 = c^2 Look at your sketch, we want to find b (x in the sketch), so subtract a^2 from both sides and we get:
b^2 = c^2 - a^2
Let's rewrite it with information from your sketch:
x^2 = 25^2 - 15^2
x^2 = 625 - 225
x^2 = 400
sqrt(x^2) = sqrt(400)
x = 20