I think I need some help.

We are performing a pre-post analysis. it is believed that the average of the variable Y is 60 in pre-situation (Y1) and it is believed to become 55 in post-situation (Y2).

we Define a new variable Y = Y2 - Y1

if Y is observed and known to be Y=-3.11, with a normal distribution and standard deviation of 1.53, what is the level of significant achieved?

I believe mu = Y2-Y1 = 55-60 = -5 is this correct?

if we calculate z = (Y-mu)/sigma = (-3.11-(-5))/1.53 = 1.23 and the corresponding probability from table is 89.07%

are my calculations and probabilities right?

What can I say in this situation? what can I claim? what does the "89.07%" show?