x=√3+√2 , y=√3-√2

3x²-5xy+3y²
=(3x²-6xy+3y²)+xy
=3(x²-2xy+y²)+xy
=3(x-y)²+xy
=3(√3+√2-√3+√2)²+(√3+√2)(√3-√2)
=3×(2√2)²+√3²-√2²
=3×8+3-2
=24+1
=25