The answer is D, 75°.
All the interior angles of a triangle add up to 180°. With this information, we can set up this equation:
(x + 5) + (3x) + (3x) = 180
Combine like terms.
(x + 5) + (3x) + (3x) = 180
7x + 5 = 180
-5 -5
7x = 175
/7 /7
x = 25
Now, since this is an isosceles triangle, we can easily substitute the value of x into 3x. 3 • 25 = 75 which is 75°.
