Could you help me with explanations, thank you

For Problems $27-36,$ evaluate $\frac{f(b)-f(a)}{b-a}.$

$27.f(x)=4x$

$28.f(x)=-3x$

$29.f(x)=5x3$

$30.f(x)=6x-5$

$31.f(x)=x^2$

$32.f(x)=3x^2$

$33.f(x)=x^{2}1$

$34.f(x)=x^2-3$

$35.f(x)=x^2-3x4$

$36.f(x)=x^{2}4x-7$