This is algebra and rearranging equations.
Let's start with question 3. The sum of all the angles in the triangle is 180 degrees. Therefore, 45 + 2k + k = 180. We can solve this:
180 - 45 = 3k by rearranging the equation so 135 = 3k, so k = 135/3 = 45 degrees. We can now see that the triangle is isosceles, with two angles being both 45 degrees.
The other two follow the same process:
4: a + 2a + 2a+ a = 360 so 6a = 360
a = 360/6 = 60 degrees
5: b + 3/2 b+ (b+45) + (2b-90) + 90 = 540
5b + 1/2b + 45 = 540 (because the -90 and +90 cancel each other out)
so 5 1/2b = 495
b = 90
If you use each letter and substitute in the values into the angles, you will find that they all add up to the sum of angle measures of each shape.
Hope I helped!