答：（1）x^2-14x=851
（x-7）^2=900
x-7=±30
x=37 或x= -23
(2) 3x^2-9x+5=0
x^2-3x+5/3=0
(x-3/2)^2=7/12
x-3/2=±√21/6
x=3/2+√21/6 或 x=3/2-√21/6