Answer
$d=-5$ or $d=3$.
Work Step by Step
By factorization, we have
$$d^2 + 2d-15=(d+5)(d-3)=0.$$
Then we have
$$(d+5) =0, \quad \text{or} \quad (d-3)=0$$
that is $d=-5$ or $d=3$.
Elementary Technical Mathematics
by Nelson, Robert; Eren, Dale;
Published by
Brooks Cole
ISBN 10:
1285199197
ISBN 13:
978-1-28519-919-1
Chapter 11 - Section 11.1 - Solving Quadratic Equations by Factoring - Exercise - Page 364: 4
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