Answer
$49$.
Work Step by Step
The given expression is
$=16y^2-56y+\_$
The standard form of the perfect square trinomial is
$=a^2-2ab+b^2$
Write the given expression in standard form.
$=(4y)^2-2(4y)(7)+\_$
So the value of $b$ is $7$.
The missing term is $b^2=7^2=49$.
