The mass \(dm\) of an element of the plate (vertical strip) will be the product of the mass density \(\rho(x)\) and the area \(A(x)\) of the strip:
[MATH]A(x)=2\sqrt{4^2-x^2}\,dx[/MATH]
[MATH]dm=2\sqrt{4^2-x^2}\rho(x)\,dx[/MATH]
Hence:
[MATH]m=2\int_0^4 \sqrt{4^2-x^2}\rho(x)\,dx[/MATH]
Can you proceed?