x = number of $4 stones
y = number of $6 stones
Ok, we know that she wants to give stones to at least 4 friends so we can set up our first inequality already:
x+y >= 4
(>= is greater than or equal to and <= is less than or equal to)
We also know she is restricted by the $30 dollar limit so we can set up the second inequality:
4x + 6y <= 30
With a little algebraic manipulation you can get them into the y=mx+b
y >= 4 - x
y <= 5 - 2/3x