Given \(P\left(x\right)+3P\left(2\right)=5x^2\left(\forall x\right)\). Calculate P(3)

With x=2 then \(4P\left(2\right)=5.2^2=20\Rightarrow P\left(2\right)=5\)

With x=3 then \(P\left(3\right)+3P\left(2\right)=5.3^2=45\Rightarrow P\left(3\right)=45-5.3=30\)