I found the picture. It is shown that the length of the base is named x and the height is 7 in.
So, now you can use the formula of the area of the triangle:
area = base * height / 2
=> 84 in^2 = x * 7 / 2
And you can solve for x:
x = 2 * 84 in^2 / 7 in = 24 in
Answer: 24 in