Let's let v1 = <a,b>, and v2 = <c,d>
v1 + v2 = <a+c, b+d> = <-3, 3>
a+c = -3
b+d = 3
Draw <-1, 0> and you see that it is horizontal. Any vector parallel will also be horizontal and have 0 for it's y value.
So, b = 0
Likewise, any vector perpendicular to <-1, 0> will be vertical and have 0 for it's x value.
So, c = 0
a and d follow