The sum of the base and height of a triangle is 12 cm. What is the largest area that the triangle can have? (see figure)

SOLUTION
It is given that the sum of the base and is 12
It follows:
[tex]\begin{gathered} x+h=12 \\ h=12-x \end{gathered}[/tex]The area of the triangle is:
[tex]A=\frac{1}{2}xh[/tex]Substitute h=12-x into the equation
[tex]\begin{gathered} A=\frac{1}{2}x(x-12) \\ A=\frac{1}{2}x^2-6x \end{gathered}[/tex]