Lisanne,
I think if you break it into the individual steps of how you get there, it might be easier to understand.
x - (1-5) = 3 (solve the parens)
x - (-4) = 3 (multiply a negative 1 times a negative # = positive #)
x + 4 = 3 (subtract 4 from both sides)
x = 3 - 4 = -1