Can anyone teach me how to calculate this surface area ?

we can calculate it by breaking it into small pieces .

there are multiple rectangular sections. calculate area of each one and add all together to get final one.

see the attached image

total surface area = area of section 1,2,3,4,5,6,7,8 and 9

you can calculate dimension of each section

section 1 - 4×2

section 2 - 3×1

section 3 - 2×1

ection 4 - 2×3

section 5 - 3×2

section 6 - 1×3

ection 7 - 1×3

section 8- 1×3

section 9 - 1×3

so total area is = 8+3+2+6+6+3+3+3+3

= 37 square unit

answer: 94

step-by-step explanation: 79+91+92+94+94 = 450 divided by 5 (since there are 5 number that are being added) = 90