The height attained by an objects thrown from ground level is given by the equation:
[tex]v^2=u^2-2gs[/tex]
At maximum height, v = 0, we assume [tex]g = 32 ft/s^2[/tex] then we have:
[tex]0=u^2-2(32)(49)\\ \\u^2=3,136\\ \\ u= \sqrt{3,136} =56[/tex]
Therefore, the minimum speed that the stone should be thrown so as to reach a height of 49 feet is 56 feet per second.