The terms of an AP are
\(T_n=a+(n-1)d\)
a is the first term and d is what keeps getting added to give the next term
for example
if a = -3 and d=5 the sequence would be -3, 2, 7, 12 etc see it starts at -3 and 5 keeps getting added.
With yours a is a positive integer and to make it easy d can be too
so the first 10 the sequence is
a, a+d, a+2d, .......a+9d
Add all those terms up and you get 10a+ 1d+2d+3d+4d+5d+6d+7d+8d+9d = 10a+ 45
What can be factorized out of that?