Try out this website or search "surface area of a triangular prism" in Google
http://www.ck12.org/geometry/Surface-Area-of-Triangular-Prisms/lesson/Surface-Area-of-Triangular-Pri...
Answer:
f your number is x, then the difference between that number and 8 would be x-8. Twice that is 2(x-8)
Three times the sum of the number and 3 would be 3(x+3). So, you get the equation:
2(x-8)=3(x+3)
2x-16=3x+9
x=-25
