Answer
$x=-17,3$
Work Step by Step
When taking the square root of a number, you get a positive and a negative answer.
$4(x+7)^2=400$
$(x+7)^2=100$
$x+7=10$
$x=3$
or
$x+7=-10$
$x=-17$.
So
$x=-17,3$.
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