(5d + 5) - (d +1)

Answer 1

5 d + 5 - d - 1

4 d + 4

Answer 2

(5d + 5) - (d +1)
= 5d + 5 - d - 1
= 4d + 4
or
= 4(d+1)

Answer 3

to convince yourself pick some number for d, like 12

(60+5) - (12+1)

65 - 13

52

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now is that the same as
4(12) + 4 ????

48 + 4

52 sure enough