Answer
$53$
Work Step by Step
RECALL:
(1) $(a+b)(a-b) = a^2-b^2$
(2) $i^2=-1$
Use rule (1) above with $a=2$ and $b=7i$ to obtain:
$=2^2 - (7i)^2
\\=4 - 49i^2$
Use rule (2) above to obtain:
$=4 - 49(-1)
\\=4-(-49)
\\=4+49
\\=53$