From the fact that the shape is a square, we know that one of the angles of the triangle in question must by 90 degrees (the lower left angle in this case).
The triangle formed is also an isosceles triangle and therefore the two smaller angles must be equal to each other. (upper left and lower right)
On top of this, we also know that the total of the angles must be 180 due to the fact that all triangles have interior angles that add to 180.
90 + (lower right) + (upper left) = 180
Since lower right and upper left are equal, we can use a single variable to represent them both.
90 + a + a = 180 90 + 2a = 180 2a = 90 a = 45 So, the degree of the lower right angle is 45. We can take this value and set 4x + 17 equal to it.
