Given data:
The given expression is an=-3+5(n-1).
The given expression can be written as,

Thus, the explicit formula for the function is F(n)=-8+5n.
Y = kx
Plug in what we know:
5 = k(8)
5 = 8k
Divide 8 to both sides:
k = 0.625
Plug this back into the equation along with y = 15:
y = kx
15 = 0.625x
Divide 0.625 to both sides:
x = 24