5.5 = -2 + d
what's the value of d? i dont know if i should add or subtract 2 because it is a negative number.

The value of d is 7.5.

The key to solving equations is doing the inverse, or opposite, function/process.
For example, you subtract when d + 1 = 2:
d + 1 = 2
(d + 1) -1 = (2) - 1
d = 1

In the same way, you add values when you have a negative:
5.5 = -2 + d = d - 2
5.5 = d - 2
(5.5) + 2 = (d - 2) + 2
7.5 = d

Now if instead of adding 2, you subtract 2, the problem becomes:
5.5 = d - 2
(5.5) - 2 = (d - 2) - 2
3.5 = d - 4

This doesn't solve your problem, it just makes the value needed to be canceled out larger.

Thus d = 7.5 and you will add because the value is negative.