All statements are true, but if you try to reverse them they won't necessarily be true.
Let's try:
Square ⇒ Has 4 sides
But if something has 4 sides it doesn't mean it's a square (example: rectangle)
Angle is 80° ⇒ acute angle
But acute angle is any angle below 90°, not just 80° so the reverse isn't true.
x – 17 = 4 ⇒ x = 21
And if we reverse
x = 21 ⇒ x – 17 = 4
we see that it's true for either direction.
x = 7 ⇒ |x| = 7
But |x| = 7 doesn't mean that x has to be 7, x can also be -7, so the reverse isn't true.
ANSWER: x – 17 = 4, then x = 21
