To do this you have to fill in the equation [tex]base*height=area[/tex], but you don't know the height, so we'll use [tex]h[/tex] for height. Plugging in the numbers you gave me, the equation is: [tex]8*h=30[/tex]. To solve this, we need to find [tex]h[/tex]. To find height you do [tex] \frac{area}{base} [/tex]. Doing this, we get [tex]h[/tex] = 3.75 ft.
Hope this helped!