y=7x^2-2√7x+2/3
=7(x^2-2√7x/7+2/21)
=7(x^2-2√7x/7+1/7-1/21)
=7(x^2-2√7x/7+1/7)-1/3
=7(x-√7/7)^2-1/3
∴当x=√7/7时,y有最小值-1/3