Algebra

First, let's put y in terms of x to make our lives easier. 

We have

$y=25-x$

Now, we subsitute it into the second expression and see what we get, 

We have

$6x+3(25-x)\\ 6x+75-3x\\ 3x+75$

Now, since x must be nonegative, the smallest value it can be is 0. Thus, we have

$3(0) + 75\\ 75$

So 75 is our answer. 

Thanks! :)