The mean is the sum of the values divided by the amount of values.
So the mean is represented here by
[tex]\frac{12+17+15+x}{4}=13[/tex]
Solve for [tex]x[/tex]:
[tex]\frac{12+17+15+x}{4}=13[/tex]
[tex]\rightarrow\frac{44+x}{4}=13[/tex]
[tex]\rightarrow 44+x=52[/tex]
[tex]\rightarrow x=8[/tex]
