Subsituting "dx" and the expression for u yields:
4x
(3x
2
+3)
1/2
dx
=
4x
u
1/2
1
6x
du
=
4 u
1/2
1
6
du
(canceling the x)
=
2
3
u
1/2
du
(slipping the constant outside and reducing to lowest terms)
4x (3x2+3)1/2 dx4x u1/2
4 u1/2
u1/24x (3x2+3)1/2 dx