In the question you provided above what do you mean can you please give more details.

Notice that Given that
is an ellipse, consider a conversion to polar coordinates:

The Jacobian for this transformation is

with determinant 
Then the integral in polar coordinates is

where you can evaluate the remaining integral by substituting
and
.
Answer:
Step 2: 20x = 80 + 10x
Step-by-step explanation:
Step 1: 20x - 10 = 80 + 10x
Add 10 to each side
20x - 10+10 = 80+10 + 10x
20x = 90 + 10x
Step 2: 20x = 80 + 10x This has the first error
Step 3: 10x = 80
Step 4: x= 8