Assume that the number of granola bars is x and the number of bottles is y
Since they decided to purchase at least 50 bottles
At least means greater than or equal, so

Since the cost of 1 bar = $0.50, then
The cost of all bars = 0.50x
Since the cost of 1 bottle = $0.75, then
The cost of all bottles = 0.75y
Since their budget is $75
They can not exceed that, then

The solution will be in the common part of the two colors
The red represents the second inequality
The blue represents the first inequality