Answer:
1) The vertex form of
is
, 2) The stone reaches its maximum height 2 seconds after being thrown.
Step-by-step explanation:
1) Given that height of the stone is represented by a second-order polynomial, which depicts a parabola as graph. The best approach to determine the instant when stone reaches its highest is by vertex form, whose form is:

Where:
,
- Instant and maximum height of the stone, measured in seconds and meters.
- Vertex constant, which must be negative as there is an absolute maximum, measured in meters per square second.
Let be
, which is transformed into vertex form:
i)
Given
ii)
Distributive property/
iii)
Existence of additive inverse/Definitions of addition and subtraction
iv)
Distributive property/
/Perfect square binomial
v)
Compatibility with addition/Existence of additive inverse/Modulative property/Definition of subtraction/Result
The vertex form of
is
.
2) The time can be extracted from previous results, which indicates that stone reaches its maximum height 2 seconds after being thrown.