im pretty sure when you change the sign in an equation you actually have to change all of them so the equation would be 3+5+7. Am i correct?

Answer:
1
Step-by-step explanation:
Mistake in the second step.
We must add positive numbers or negative ones.
Here he had add the positive and negative numbers and that is the mistake
[tex]=3-(-5)-7\\\\=3+5-7[/tex]Here 5 is positive because -×-=+
[tex]=8-7[/tex]
[tex]=1[/tex]
In the second step, we must add the positives that are 3 and 5 and subtract from 7.
Hope it will help you.