Question 964264
b=base; h=height=b-2cm
{{{A=(1/2)bh}}} Substitute for h.
{{{A=(1/2)(b)(b-2cm)}}}
{{{28cm^2=(1/2)(b)(b-2cm)}}} Multiply each side by 2.
{{{56cm^2=b^2-2b}}} Subtract 56 from each side.
{{{0=b^2-2b-56}}}
*[invoke quadratic "b", 1, -2, -56 ]
ANSWER 1: The base is 8.55 cm. (This would round to 8.5cm- kept extra digit for check)
h=b-2cm=8.55cm=6.55 cm (see above) 
ANSWER 2: The height is 6.55 cm.
CHECK:
{{{A=(1/2)bh}}}
{{{28cm^2=(1/2)(8.55cm)(6.55cm)}}}
{{{28cm^2=(1/2)(56.00cm^2)}}}
{{{28cn^2=28cm^2}}}