Question 202423
To find the domain of a function, it is most appropriate to create a graph. Domain  satisfies the bounds for the x-axis, or the horizontal movement of the graph. 

M(x)=5/x^2 - 9 is a rational function with the following graph: {{{ graph( 300, 200, -20, 20, -20, 20, 5/x^2 - 9) }}}, so the domain is (-∞,∞).

L(x)=5x-4 is a linear equation with the graph: {{{ graph( 300, 200, -20, 20, -20, 20, 5x-4) }}}. As seen in the graph, it is a line that has no bounds in the Cartesian Coordinate plane. 
Its domain is (-∞,∞).

G(x)=7x=4/x=4. If you can clarify the equation I can help you, but this is clearly not a function since there are three equal signs.