small question i was find confused if i shloud replace mx or y int

SOLUTION:
The question says that the cost of 25 cubic meters of flagstone is $350.
This is just a proportional relationship. Thus ;
[tex]y=mx,\text{ }b=0[/tex]Since the cost of 0 cubic meters of flagstone should be zero.
Thus, lets find m;
[tex]\begin{gathered} 350=25m \\ m=14 \end{gathered}[/tex]Thus, the amount of flagstone that can be purchased for $500 is;
[tex]\begin{gathered} 500=14x \\ x=\frac{500}{14}=35.7 \end{gathered}[/tex]Thus, 500 would fetch us 35.7 cubic meters of flagstone