Answer:
See figure attached.
Step-by-step explanation:
Let's define:
x: internet usage, in GB
y: cost, in $
Between 0 GB and 10 GB, the cost is always $50.
Between 10 GB and 25 GB, cost is a line with slope = $4 per GB which pass through point (10, 50), the last point of the previous segment.
From 25 GB on, cost is a line with slope = $3 per GB which pass through point (25, 110), the last point of the previous segment.