The slope of a line can be calculated with the following formula:
[tex]m=\frac{y_2-y_1}{x_2-x_1}[/tex]You can notice that it is the difference between the y-coordinates and the difference between the x-coordinates of two points on that line.
For this case, knowing these points:
[tex]\begin{gathered} (7,15) \\ \mleft(0,15\mright) \end{gathered}[/tex]You can say that:
[tex]\begin{gathered} y_{2\text{ }}=15 \\ y_1=15 \\ x_2=0 \\ x_1=7 \end{gathered}[/tex]Now you can substitute values into the formula and the evaluate:
[tex]\begin{gathered} m=\frac{15-15}{0-7} \\ m=\frac{0}{-7} \\ m=0 \end{gathered}[/tex]Therefore, the slope is 0:
[tex]m=0[/tex]