You can do these by two Methods.
1) Splitting middle term
2) Quadratic formula
I am doing with first method
6)x^2-6x+8=0
x^2-(4x+2x)+8=0
x^2-4x-2x+8=0
x(x-4)-2(x-4)=0
(x-4) * (x-2) =0
x= +4 or +2
7) x^2-9x/4-25=0
4x^2-9x-100=0
D=√b^2-4ac
D= √81-4*4*-100
D√1681
D= 41
X= 9+ 41/ 8 OR 9-41/8
X= 50/8 OR -4