You are asked to complete the square on $\simplify{x^2+{b}x + {c}}$
The exponent of x is 2, so we halve it to get $\var{p}$.
Squaring this gives $\var{p^2}$.
We can write
\[\simplify{x^2+{b}x + {c}} = \simplify[all, !collectNumbers]{x^2+{b}x + {p^2} - {p^2} + {c}}\]
The first three terms are a perfect square, so we have
\[\simplify{x^2+{b}x + {c}} = \simplify[all,!collectNumbers]{(x+{p})^2 - {p^2} + {c}}\]
Leading to
\[\simplify{x^2+{b}x + {c}} = \simplify{(x+{p})^2 - {p^2} + {c}}\]