Part a
p2 – 6p + 9
we know that
p2 – 6p + 9=(p-3)^2
so
the given expression is a square of linear equation
Part b
9x2 – 36=(3x-6)(3x+6)
the given expression is not a square of linear equation
Part c
x2 + 6x + 36
the given expression is not a square of linear equation
Part d
(1/2x+4)^2
he given expression is a square of linear equation
