Why is 4X + 5 a linear function

Why is y=X2 a nonlinear function

y = 4x+5 is in the form y = mx+b which is slope intercept form. Anything in y = mx+b form is linear. It graphs out a straight line.

Contrast this with y = x^2 which is nonlinear because it graphs a curve that isnt a straight line. All quadratics like this graph out a parabola which resembles a bowl shaped curve.