The value of x is B. 70°.
The angles of a triangle always add up to 180°. First, you must subtract 40 from 180 to know the sum of the remaining angles.
180 - 40 = 140 ⇒ The sum of the remaining angles is 140°
The Isosceles triangle theorem states that if two sides of a triangle are congruent, then the angles opposite those sides are congruent. Since the angle measurements of the remaining two angles are congruent, to find the value of x, you have to divide the sum of the remaining angles by 2.
140/02 = 70 ⇒ The value of x is 70°