ANSWERS
...The slope of FG and IH was 4/5. The slope of FI and GH was -5/4. Then I knew that ∠F, ∠G, ∠H, and ∠I were all right angles because perpendicular lines have opposite reciprocal slopes.
Step-by-step explanation
The slope of a line is given by,

For lines FG and IH, the slope is,

Similarly, the slope of lines FI and GH is,

Note that the slopes are opposite reciprocals, which makes each pair of lines perpendicular lines.