Step 1 :First, let's find the radius of the hemisphere and the base of the cone by dividing the diameter by 2: \(r = \frac{9}{2} = 4.5\) cm.
Step 2 :Next, let's find the volume of the hemisphere using the formula \(V_{hemisphere} = \frac{2}{3} \pi r^3\): \(V_{hemisphere} = \frac{2}{3} \pi (4.5)^3 = 60.75\pi\) cm³.
Step 3 :Let's find the volume of the cone using the formula \(V_{cone} = \frac{1}{3} \pi r^2 h\), where h is the height of the cone.
Step 4 :Since the total volume of the solid T is the sum of the volumes of the hemisphere and the cone, we can set up an equation to solve for y, the total height of the solid T: \(\frac{2}{3} \pi r^3 + \frac{1}{3} \pi r^2 h = 140\pi\).
Step 5 :Plug in the values we know: \(60.75\pi + \frac{1}{3} \pi (4.5)^2 h = 140\pi\).
Step 6 :Solve for h: \(h \approx 11.741\) cm.
Step 7 :Finally, add the height of the cone to the radius of the hemisphere to find the total height of the solid T: \(y = h + r \approx 11.741 + 4.5 = 16.241\) cm.
Step 8 :\(\boxed{y \approx 16.241}\) cm (correct to 3 significant figures).