You're looking for a solution of the form
Differentiating twice yields
Substitute these series into the DE:
Two of these series start with a linear term, while the other two start with a constant. Remove the constant terms of the latter two series, then condense the remaining series into one:
which indicates that the coefficients in the series solution are governed by the recurrence,
Use the recurrence to get the first few coefficients:
You might recognize that each coefficient in the <em>n</em>-th position of the list (starting at <em>n</em> = 0) involving a factor of -7 has a denominator resembling a factorial. Indeed,
-7 = -7/0!
-7/2 = -7/2!
-7/6 = -7/3!
and so on, with only the coefficient in the <em>n</em> = 1 position being the odd one out. So we have
which looks a lot like the power series expansion for -7<em>eˣ</em>.
Fortunately, we can rewrite the linear term as
3<em>x</em> = 10<em>x</em> - 7<em>x</em> = 10<em>x</em> - 7/1! <em>x</em>
and in doing so, we can condense this solution to
Just to confirm this solution is valid: we have
<em>y</em> = 10<em>x</em> - 7<em>eˣ</em> ==> <em>y</em> (0) = 0 - 7 = -7
<em>y'</em> = 10 - 7<em>eˣ</em> ==> <em>y'</em> (0) = 10 - 7 = 3
<em>y''</em> = -7<em>eˣ</em>
and substituting into the DE gives
-7<em>eˣ</em> (<em>x</em> - 1) - <em>x</em> (10 - 7<em>eˣ </em>) + (10<em>x</em> - 7<em>eˣ</em> ) = 0
as required.