Question 1006190
check for the formulas in your book; but t is for term and s is for a summation.  You have arithmetic sequence and series.  This help is for your part (a).


t1+(n-1)d=tn, excuse the poor subscripting; and you have  t1+(3-1)d=6.5.


Next facts give you s=(n/2)(t1+tn) but I really should show this to you better as
{{{s[8]=(8/2)(t[1]+t[8])}}}, and you were given the value of the summation,
{{{s[8]=(8/2)(t[1]+t[8])=67}}}.


You will have two equations in the two unknowns {{{t[1]}}} and d.