I want to plot3d this function :
(2*Int(3*piecewise(x < 1, x, 1 < x, 2 - x)*piecewise(y < 3/2, y, 3/2 < y, 3 - y)*sin(Pi*x/2)*sin((2*Pi*y)/3), [y = 0 .. 3, x = 0 .. 2]))/3
 

So, I wrote : 

plot3d((2*Int(3*piecewise(x < 1, x, 1 < x, 2 - x)*piecewise(y < 3/2, y, 3/2 < y, 3 - y)*sin(Pi*x/2)*sin((2*Pi*y)/3), [y = 0 .. 3, x = 0 .. 2]))/3, x = 0 .. 2, y = 0 .. 3)

and I get :

Warning, unable to evaluate the function to numeric values in the region; see the plotting command's help page to ensure the calling sequence is correct

I am totally new with integrals and I think there is a pb related to a bad writing of them.
Thank you for your answers.

I am trying to solve (unsuccessfully) the title equation with MAPLE 2020.1

If someone could help me...

Here is my code, inspired by the above code :)

restart;
with(plots);
with(PDEtools);
a := 1;
b := 1;
rho := 8166;
h := 10^(-3);
Dp := 19.2615;
f := sin(7*x)*cos(4*y);
WavEq := [rho*h*diff(w(x, y, t), t $ 2)/Dp + diff(w(x, y, t), x $ 4) + 2*diff(w(x, y, t), x $ 2, y $ 2) + diff(w(x, y, t), y $ 4) = 0, u(0, y, t) = 0, u(a, y, t) = 0, u(x, 0, t) = 0, u(x, b, t) = 0, u(x, y, 0) = f];
sol := pdsolve(WavEq);

This get the following error message : "Error, (in pdsolve/sys) the input system cannot contain equations in the arbitrary parameters alone; found equation depending only on _F1(y,t): _F1(y,t)"