Варифметической прогрессии a7=20,a10=11 найдите a19

A7=20   a10=11  a19-?
a19=a1+18d

a7=a1+6d
a10=a1+9d
20=a1+6d
11=a1+9d
9=-3d
3d=-9
d=-3
a7=a1+6d
20=a1+6*(-3)
20=a1-18
a1=-18-20
-a1=-38
a1=38
a19=38+18*(-3)
a19=38-54
a19=-16