Solve the following math problem efficiently and clearly:

- For simple problems (2 steps or fewer):
Provide a concise solution with minimal explanation.

- For complex problems (3 steps or more):
Use this step-by-step format:

## Step 1: [Concise description]
[Brief explanation and calculations]

## Step 2: [Concise description]
[Brief explanation and calculations]

...

Regardless of the approach, always conclude with:

Therefore, the final answer is: $oxed{answer}$. I hope it is correct.

Where [answer] is just the final number or expression that solves the problem.

Problem: A frustum of a right circular cone is formed by cutting a small cone off of the top of a larger cone. If a particular frustum has a lower base radius of 6 inches, an upper base radius of 3 inches, and a height of 4 inches, what is its lateral surface area?  (The lateral surface area of a cone or frustum is the curved surface excluding the base(s).)

[asy]size(200);
import three; defaultpen(linewidth(.8)); currentprojection = orthographic(0,-3,0.5); pen dots = linetype("0 3") + linewidth(1);
real h = 2.3, ratio = (91-24)/(171-24);
picture p1, p2; /* p1 is left-hand picture */
triple A = (0,0,0), B = (0,0,h); draw(p1,(-1,0,0)..(0,-1,0)..(1,0,0)); draw(p1,(-1,0,0)..(0,1,0)..(1,0,0),dots); draw(p1,(-1,0,0)--B--(1,0,0));
add(p1);

triple vlift = (0,0,0.5);

path3 toparc1 = shift((0,0,h*(1-ratio)))*scale3(ratio)*((-1,0,0)..(0,1,0)..(1,0,0)), toparc2 = shift((0,0,h*(1-ratio)))*scale3(ratio)*((1,0,0)..(0,-1,0)..(-1,0,0));
draw(p2,(-1,0,0)..(0,-1,0)..(1,0,0)); draw(p2,(-1,0,0)..(0,1,0)..(1,0,0),dots);

draw(p2,(-1,0,0)--ratio*(-1,0,0)+(1-ratio)*B^^ratio*(1,0,0)+(1-ratio)*B--(1,0,0));

draw(p2,shift(vlift)*(ratio*(-1,0,0)+(1-ratio)*B--B--ratio*(1,0,0)+(1-ratio)*B));

draw(p2,toparc1--toparc2); draw(p2,shift(vlift)*toparc1,dots); draw(p2,shift(vlift)*toparc2);

draw(p2,shift(vlift)*((1-ratio)*B--B),linewidth(0.7)); dot(p2,shift(vlift)*((1-ratio)*B),linewidth(1.5));
label(p2,"frustum",(0,0,h/4));

add(shift((3.4,0,0))*p2);

[/asy]
user