Change in momentum?

If change in momentum is mv-mu, and let's say we have a ball of mass 1 kg bouncing off a wall with u = 3 and v = -4 m/s. Now, change in momentum would be -4 - 3 = -7. But isn't this wrong? Shouldn't the change be positive?

No. It had positive momentum and now has negative momentum so the change is negative.

Ah, ok, a certain question confused me:

q2 here

http://theallpapers.com/papers/CIE/AS_and_ALevel/Physics%20(9702)/9702_s09_qp_2.pdf

It's a similar situation. The ball is initially travelling in one direction and bounces back. You can calculate the momentum change in the same way as last time.

the MS says that the answer must be positive