The formula of an area of a triangle
[tex]A=\dfrac{bh}{2}[/tex]
We have [tex]b, h = b + 3, A = 35 ft^2[/tex]
Substitute:
[tex]\dfrac{b(b+3)}{2}=35\ \ \ |\cdot2\\\\b^2+3b=70\ \ \ |-70\\\\b^2+3b-70=0\\\\b^2+10b-7b-70=0\\\\b(b+10)-7(b+10)=0\\\\(b+10)(b-7)=0\iff b+10=0\ \vee\ b-7=0\\\\b=-10 < 0\ \vee\ b=7[/tex]
The altitude [tex]b+3=7+3=10\ ft[/tex]
Answer: 10ft.
