HI aspirant,

S=u+1/2a(2n-1)
10=u+1/2a(7)...(1)
15=u+1/2a(11)...(2)
Subtract eq(1)&(2)
a=5/2m/s^2
U=5/4m/s
After next third second
S9=5/4+1/2×5/2(2×9-1)
S9=22.5m

hence is the answer. you must use this formula to obtain the answer. with this base formula you can solve n number of questions asked in n number of ways. be clear in concepts

all the best