The expression is:
[tex]y=-7x+8[/tex]To be a function this has to have a unique values of y for each values of x, son in this case the expression has the form of a linear function that is equal to:
[tex]y=mx+b[/tex]So So for example if i replace x = 1:
[tex]\begin{gathered} y=-7(1)+8 \\ y=1 \end{gathered}[/tex]There is not other number that I can give to x that the output of y will be equal to 1, this means that the expression y = -7x + 8 is a function