In a rectangle diagonals is equal (|AC| = |BD|), therefore
2(5a + 1) = 2(a + 1) |divide both sides by 2
5a + 1 = a + 1 |subtract 1 from both sides
5a = a |subtract "a" from both sides
4a = 0 |divide both sides by 4
a = 0
subtitute to 2(a + 1)
|AC| = |BD| = 2(0 + 1) = 2(1) = 2
