Definitely follow Richard's advice, and read up on operators - this is basic stuff so if you don't know it all pretty well, you aren't going to pass a verbal interview even if you hand in a perfect result for this task!
Then write each bit with parentheses round it:
1 + 2
(1 + 2) - 1
((1 + 2) - 1) * 2
(((1 + 2) - 1) * 2) / 2
((((1 + 2) - 1) * 2) / 2) + 8
(((((1 + 2) - 1) * 2) / 2) + 8) % 7
Then think about it: what parentheses can you remove?
For example:
a + b - c
is the same as
(a + b) - c
and
a + (b - c)
But
a + b * c
will give you different result depending on which way you evaluate it:
(a + b) * c
is not the same as
a + (b * c)
so the parentheses are important.
Stop panicking, and think about what is required of you: this isn't a complex task so it shouldn't be difficult for you.