The lines on the sides of the triangle represent lines of equal length. This means that this triangle is an ‘isosceles’ triangle. Isosceles triangles have two equal sides and two equal angle measures on the base of each equal side.
Knowing this, the angles on the bottom left and right are equal. We also know that angles on one side of a line are supplementary: they add to 180 degrees. As such, 180 - 110 is 70 degrees - the angle measures of both the bottom left and bottom right angle, due to the triangle being isosceles.
All angles in a triangle add to 180 degrees as well. We can find the missing angle by subtracting the two angle measures already found: 180 - 70 - 70 = 40
