Question 1:
Since this is a rectangle, the diagonals must be congruent.
8x + 3 = 4x + 7
Subtract both sides by 4
4x + 3 = 7
Subtract both sides by 3
4x = 4
Divide both sides by 4
x = 1
Question 2:
The first choice is incorrect because that could satisfy for a rhombus.
The third choice is incorrect because that could satisfy for any parallelogram.
The last choice is incorrect since the first and third choices don't work.
The second choice is the correct answer due to elimination and also because it's the only method that works.
Question 3:
First, which shapes have perpendicular diagonals?
Only rhombuses and squares do. We can eliminate everything else.
Which shapes have congruent diagonals?
Only rectangles have congruent diagonals.
A square is a type of rectangle. Thus, a square is the correct answer.
Have an awesome day! :)