Answer:


Step-by-step explanation:
Given


Required
Solve
Substitute
in 

Open bracket

Collect like terms


Divide both sides by 2

Substitute
in 



Answer:
x=-1
y=4
z=7
Step-by-step explanation:
3x + y - z= -6(1)
2x - y + 2z = 8(2)
4x + y - 3z = -21(3)
(1)+(2): 5x+z=2(4)
(2)+(3): 6x-z=-13(5)
(4)+(5): 11x=-11
x=-1
so z=2-5x=7
y=-6-3x+z=4
I and III seem to be correct, II doesn't because, for example, if x = w = z = t = 70 and y = u = 40, then all three pairs of vertical angles in the figure have equal measure and the given condition x + y = u + w stands. But it means y is not equal to w.
Let's begin noting that a triangle is isosceles if and only if two of its angles are congruent. We can thus find the angle <ABP, recalling that the sum of the interior angles of a triangle is equal to 180°.

Finally, let point K be the intersection between segments BC and PQ, and let's note that the triangle PQB is a right isosceles triangle, since all the angles in a square are equal to 90°, and the two triangles APB and BQC are congruent.
Therefore, the angle BKQ is equal to 180-50-45=85°.
Of course angle BKP=180-85=95°.
Hope this helps :)