Answer:
15
Step-by-step explanation:
Sum of 'n' terms formula is given by:-
s=n/2(2a+(n-1)d)
s=13/2[2xa+(13-1)5]
s=13/2(2xa+12x5)
s=13/2(2a+60)
585=13/2(2a+60)
585 x (2/13) = 2a + 60
90 = 2a + 60
90-60 = 2a
30 = 2a
a = 15
Factoring by grouping usually pairs up the first 2 sets of expressions with the second 2 sets. Ours looks like this, then:

. If we factor out the common x-squared in the first set of parenthesis, along with factoring out the common 5 in the second set, we get this:

. Now the common expression that can be factored out is the (x-9). When we do that, here's what it looks like:

. I'm not sure how far you are going with this. You could set each of those equal to 0 and solve for x in each case. The first one is easy. If x - 9 = 0, then x = 9. The second one involves the imaginary i since x^2 = -5. In that case,

. Hopefully, in what I have given you, you can find what you're looking for.