on the substitute 3 into 1 part, how does it become 2x²?

You multiply all terms on both sides by the x from the denominator of 40/x.  This allows you to remove the variable from the denominator; in the process the first term becomes squared because 2x * x = x^2
2x+2y=26

y=40/x

2x+2(40/x)=26

Divide both sides by 2:

x+40/x=13

Multiply both sides by x:

x^2 +40=13x

Rearrange the terms:

x^2 -13x+40=0
The x from 2 ( 40/x ) goes to the both side.
How did the 2x from above become 2x²?. Doest it have to do with the change into quadratic equation thing or am I missing something
Multiply both sides by x to get rid of the fraction

Which gives you a nice polynomial to work with
It becomes 2x^2 after he multiplys both sides by x... Thereby eliminating x as the denomination of 40 /x...
Multiply each term on both sides of the equation by: X/1

Practice writing out the X/1 multiplied by each term on both sides.

Do each multiplication by hand. After that collect any like terms (there aren’t any)

Practice writing out any algebraic steps and completing each individual algebraic calculation.

For your information the textbook really should not have skipped this step unless it’s university calculus 1 or higher. Anything in high school should be showing steps like algebraically multiplying each term on both sides of the equation by X/1

Edit: why is this downvoted? Man screw this dumb sub I’m done trying to help random people