Answer:
B
Step-by-step explanation:
Drop an altitude. Either of the two resulting right triangles with legs of length 5 and x and a hypotenuse of 10.
By the Pythagorean theorem, it follows that the answer is B.
Your answer is 21 for this one.

Break the expression into group:

factor out x from 4x² - 5x : x (4x-5)
factor out 3 from 12x-15 : 3 (4x-5)

factor out common term (4x-5) :

hope this helps!