Question 935982
{{{A[n]=1+4(n-1)}}}, general term for the sequence.


{{{(A[1]+L)(n/2)}}}, L is for LAST term in the sequence,
The formula for sum of a sequence of n terms.
Better, the expression is {{{(A[1]+A[1]+4(n-1))(n/2)}}}.


Your sum 153, you want to find n.
{{{(1+(1+4(n-1)))(n/2)=153}}}


The first goal is to understand all that, leading to the equation just given.  The rest is to just simplify that equation and solve for n.