math problem

the equation of the oblique line is y =3x
so the integral is
(12-3x) dx with limit form 0 to 4
=(12x- 3/2 x^2) with limit form 0 to 4
=(12*4 -3/2 * 4^2) - (12*0 - 3/2 * 0^2)
=24

Please tell me if it is not clear enough

2008-03-24 15:25:40 補充：
The following is using double integral
the equation of the oblique line is y =3x
integrate from 0 to 4 [integrate from 3x to 12 (1)]dy dx
=integrate from 0 to 4 [y from 3x to 12 ]dx
=integrate from 0 to 4 (12-3x)dx
=(12x- 3/2 x^2) from 0 to 4
=(12*4 -3/2 * 4^2) - (12*0 - 3/2 * 0^2)
=24