Ugh this hurts my head just looking at it, but I'll show the process.
PEMDAS
Addition before subtraction (Not that it matters here).
2 - (-4) is the same as adding 4. This equals 6.
-1 - (-3) is the same as adding 3. This equals 2.
Adding these two expressions together equals 8.
Showing my work...
2 - (-4) + (-1) - (-3)
6 + (-1) - (-3)
6 + 2
8