​ Please answer me, its urgent

I have never really used nets before..but I think all you have to do is pretend that the pyramid is made out of paper, and you cut along the edges and lay the paper flat on the table...so for this pyramid with a triangle base, it would look sort of like:

(This isn't a very accurate drawing!)

The surface area of the pyramid is the sum of the areas of each of these triangles,

and the area of a triangle = (1/2)(base)(height)

SA = (1/2)(8)(9) + (1/2)(6)(9) + (1/2)(10)(10.2) + (1/2)(8)(6)

SA = 36 + 27 + 51 + 24

SA = 138 square cm

Looks pretty accurate to me, hectictar....!!!!.....I have to admit....the concept of "nets"  is foreign to me, too......but....it does seem like a good way to visualize exactly what is going on....