Solving system of linear equations 1. X - 2Y = 8
X = 5- Y

2. X + Y = 10
Y = X - 4

Iam unsure about what the proof is, therefore as much as wish to i can not.

If two sides of a triangle are congruent, then the angles opposite those sides are congruent. the converse of the base angles theorem, states that if two angles of a triangle are congruent, then sides opposite those angles are congruent.

It's all about the 4

The answer to your question is 1