The surface area would be the combined area of all planes. The area of the ends is (3*4)/2, or 6, because the area of a triangle is b*h/2. The area of the three surrounding planes is (12*5)+(12*3)+(12*4), or 60+36+48=144. Knowing that the length of the prism is 12, you must multiply is by each height of the prism’s base. From there, you can add 144+6+6, which gives you 156, the combined areas of all 2 dimensional planes on the 3 dimensional model, or the surface area.