at time stamp 14:58 in practice problems 16 why is he calculating n it is asked to calculate d?

Question

Nik D · Accepted Answer

Use the formula for the nth term of an AP to find the common difference:
l=a+(n&minus;1)d
Substitute the known values:&rArr;51=7+(12&minus;1)𝑑&rArr;51=7+11d
Solve for d: &rArr;51=7+(12&minus;1)d&rArr;51=7+11d ; 44=11d&rArr;d=11/4