So, I'll assume that you got it at this point. Now, for the quick way : first, calculate the area of the triangles (the bases of prism) : h*b/2 = 12*9/2 = 54 for one triangle, and since you have 2, 54*2 = 108 for both. Let's put that temporarely in the back of our minds whilst we calculate the rest
Now that this is done, you're left with the 3 rectangles. They have something special, these 3 rectangles : they all have the same height (11 in this case). So here is the total area for the 3 of them :
9*11 + 12*11 + 15*11
Since 11 is a common factor to the 3 terms, I suggest we factor it from the addition :
11(9 + 12 + 15) = 396
9+12+15, that looks a lot like the perimeter of your triangle, right? So that's how you can calculate the area of any prism : the area of both bases + (perimeter of the base * height of the prism). So in this case : 2*54 + 11*(9+12+15) = 504 m^2