#### Answer

$d = 2 \sqrt {(c - a)^{2} + (d - b)^{2}}$

#### Work Step by Step

$d = \sqrt {(2c - 2a)^{2} + (2d - 2b)^{2}}$
$d = \sqrt {(2(c - a))^{2} + (2(d - b))^{2}}$
$d = \sqrt {4(c - a)^{2} + 4(d - b)^{2}}$
$d = \sqrt {4((c - a)^{2} + (d - b)^{2})}$
$d = 2 \sqrt {(c - a)^{2} + (d - b)^{2}}$