The Force on the scale is given by,

F=ma

The regular accelration is 9.8 m/s/s

Thus, when another a m/s/s is added we have,

582=m(9.8+a)

When it stops we have,

399=9.8m

Thus,

m=40.7 kilograms (am I right?)

Thus,

582=40.7(9.8+a)

We have,

a=4.49 m/s/s

Wow, I actually did a physics problem!

(Though I did not understand anything I was doing).