Let s=number of seeds and t=number of days...
st=120 and (s+10)(t-2)=120
From the first we can say t=120/s, now using this value of t in the second equation gives us:
(s+10)(120/s-2)=120
(s+10)(120-2s)/s=120
(s+10)(120-2s)=120s
120s-2s^2+1200-20s=120s
2s^2+20s-1200=0
s^2+10s-600=0
(s+30)(s-20)=0, since s>0
s=20, and since she planted s+10 seeds per day to finish two days earlier:
20+10=30
She planted 30 seeds per day.