He is incorrect because if a number is before a letter than the number is the coefficient. and the coefficient is always multiplied by the letter its next to. since f is being distributed to x (multiplied because of the parentheses) then fx=3x which means f = 3 which means 3x=3x
Take

so that you have

which gives a Jacobian determinant of

So upon transforming the coordinates to the u-v plane, you have (and I'm guessing on what the integrand actually is)

Answer:
Step-by-step explanation:
The given triangle is a right angle triangle. The opposite side is given as 244 yards. The adjacent side is given as 119 yards. To determine the hypotenuse, we would apply Pythagoras theorem. It is expressed as
Hypotenuse^2 = opposite side^2 + adjacent side^2
Hypotenuse^2 = 244^2 + 119^2
Hypotenuse^2 = 59536 + 14161
Hypotenuse = √73697 = 271.5 yards.
cos θ = adjacent side/hypotenuse
cos θ = 119/271.5 = 0.4383
θ = Cos^-1(0.4383) = 64 degrees
The other angle would be
90 - 64 = 26 degrees