The cost to rent the trucks is fixed, it doesn't depends on the amount of sugar. While the cost for each ton depends linearly. So the equation is:
[tex]C=150\cdot S+4500[/tex]To calculate the cost to transport 13 tons of sugar, we just substitute S for the value (13):
[tex]C=150\cdot13+4500=1950+4500=6450[/tex]The cost to transport 13 tons of sugar will be $6450.