How is -5x + 2y = 9 is not constant of variation

Yes because there is no constant

A constant of variation is only in the form ay=bx where b/a=constant so given any point y/x=constant let us check here by substituting x=1 and x=2
when x=1---> 2y=14,y=7 here y/x=7
when x=2--->2y=19,y=9.5 here y/x=4.75
so this equation is not constant of variation
let us check an equation of the form 3y=7x
when x=1--->y=7/3--->y/x=7/3
when x=2--->y=14/3>y/x=7/3
so 3y=7x is a constant of variation