Explain to me how to find the missing measurement

So you find out how many triangles are in this by taking a point and making a line to another point that isn't on the same line. this one has 3 triangles. each triangle is 180*(using multiplication symbol for degree symbol).you do this to find out what the number of angles adds up to. 180* multiplied by 3 equals 540*. Then if you add up all the angles you have 400*. 540 - 400 equals 140*. So the final angle is 140*. Hope this helps!

They are somewhat similar