A formula for calculating acceleration by successive difference method in physics of grade one of senior high school (with explanation) I know seven points first, then S4-S1 s5-s2 s6-s3 And then what?

A formula for calculating acceleration by successive difference method in physics of grade one of senior high school (with explanation) I know seven points first, then S4-S1 s5-s2 s6-s3 And then what?

knowable
S4-S1=3at*t
S5-S2=3at*t
S6-S3=3at*t
It can be seen from the sum of the three formulas
S4+s5+s6-S1-S2 -S3=9at*t
therefore
a=(S4+s5+s6-S1-S2 -S3)/9t*t