Answer:
A quadratic equation is a polynomial equation whose highest degree is two.
If we substitute [tex]a=0[/tex] into the standard form of a quadratic equation [tex]y=ax^2+bx+c[/tex] then we eliminate the variable with degree 2. This means it is no longer a quadratic equation and has become a linear equation, since a linear equation is an equation in which the highest degree is one.
Proving algebraically
Substitute [tex]a=0[/tex] into [tex]y=ax^2+bx+c[/tex]:
[tex]\implies y=(0)x^2+bx+c[/tex]
[tex]\implies y=bx+c[/tex]