According to the factor theorem, if (x−a) is a factor of a polynomial f(x), then f(a) = 0.
For f(x)=x2−3x+2
f(1)=12−3× 1+2=0.
Hence, (x−1) is the factor of x2−3x+2.
For f(x)=x2−7x+12
f(3)=32−7× 3+12=0.
Hence, (x−3) is the factor of x2−7x+12.
For f(x)=x2−11x+30
f(5)=52−11× 5+30=0.
Hence, (x−5) is the factor of x2−11x+30.
For f(x)=x2−15x+56
f(7)=72−15× 7+56=0.
Hence, (x−7) is the factor of x2−15x+56.