A arithmetic sequence follows the form An=A1+(n-1)d
If the third term is equal to 5, then 5=A1+2d.
If the sum of the first 10 terms is 125, then 125=Sum(A1 +[n-1]d) from n=1 to n=10.
Therefore 125=10A1+45d (45=1+2+3+4+5+6+7+8+9)
Solve linear equations:
+ (A1+2d= 5)(-10)
Therefore d=3 and substituting into the first equation A1=-1
First term is -1 and the difference is 3.