Parameterize S in cylindrical coordinates by the vector function
r(u, v) = (x(u, v), y(u, v), z(u, v))
with
x(u, v) = u cos(v)
y(u, v) = u sin(v)
z(u, v) = 36 - u²
with 0 ≤ u ≤ 3 and 0 ≤ v ≤ 2π. Take the normal vector to S to be
[tex]\vec n = \dfrac{\partial r}{\partial u} \times \dfrac{\partial r}{\partial v} = (2u^2\cos(v),2u^2\sin(v),u)[/tex]
so that the surface element is
[tex]dS = \|\vec n\| \, du\,dv = u \sqrt{1+4u^2}\, du\, dv[/tex]
Then the surface integral is
[tex]\displaystyle \iint_S y^2\,ds = \int_0^{2\pi} \int_0^3 u\sqrt{1+4u^2} \, du\, dv[/tex]
[tex]\displaystyle \iint_S y^2\,ds = 2\pi \int_0^3 u\sqrt{1+4u^2} \, du[/tex]
[tex]\displaystyle \iint_S y^2\,ds = \frac{2\pi}8 \int_0^3 8u\sqrt{1+4u^2} \, du[/tex]
[tex]\displaystyle \iint_S y^2\,ds = \frac\pi4 \int_0^3 \sqrt{1+4u^2} \, d(1+4u^2)[/tex]
[tex]\displaystyle \iint_S y^2\,ds = \frac\pi4 \cdot \frac23(1+4u^2)^{\frac32} \bigg|_0^3 = \boxed{\frac\pi6 \left(37^{\frac32}-1\right)}[/tex]
