Answer:
7 ft
Step-by-step explanation:
[tex]h(x)=-0.5(x^2-4x-21)\\\\\text{The zeros for}\ h(x)=0:\\\\-0.5(x^2-4x-21)=0\qquad\text{divide both sides by}\ -0.5\\\\x^2-4x-21=0\\\\x^2+3x-7x-21=0\\\\x(x+3)-7(x+3)=0\\\\(x+3)(x-7)=0\iff x+3=0\ \vee\ x-7=0\\\\x+3=0\qquad\text{subtract 3 from both sides}\\x=-3<0\\\\x-7=0\qquad\text{add 7 to both sides}\\x=7[/tex]
