How do you do this type of equation? Please answer and explain?

3b+b-8=4 and also can you exLain to me like let's say theres this irregular polygon that has this triangle and a square. The question says to find the area of the polygon. I know you have to find the area of both of these and add them up but sometimes like it doesn't give you All the numbed for the height can you alsO xPlaAn to me how To find the area of an irregular polygon please o have a final exam tomorrow and if I don't pass this wil not be a great summer f