If you need to prove that a quadrilateral is a rectangle, what must you first prove?

A rectangle is a parallelogram with four right angles.
A quadrilateral is a rectangle if and only if it has four right angles

https://mathbitsnotebook.com/Geometry/Quadrilaterals/QDRectangle.html

To prove a quadrilateral is a rectangle:

1) Prove the shape is a parallelogram by doing slope 4 times by stating that parallel lines have equal slopes. Then proving a right angle by stating that perpendicular lines have negative reciprocal slopes.

--OR--

2) Doing the slope 4 times and stating that the shape is a rectangle because opposite sides are parallel because of equal slopes and it contains a right angle because of negative reciprocal slopes.

--OR--

3) Find all four slopes and find all angles are formed by intersecting lines with slopes m and n. Show that n = -1/m, so all angles are right. Therefore this is a rectangle.

http://mathcentral.uregina.ca/QQ/database/QQ.09.06/s/sonja1.html

You can also prove the parallelogram part by showing that opposite pairs of sides have equal lengths. Then showing that any one angle is a right angle is sufficient to prove that it is a rectangle.

Depending on the information available, you might just go straight to showing that the figure has 3 right angles (since the angle sum of a quadrilateral is 360 degrees, this means that the fourth angle must also be 90 degrees).

https://en.wikipedia.org/wiki/Rectangle