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This is equal to 0 = 4x^2 -8x+7 which is a parabola. I don't know what desmos is so I can't help you with that
This is the equations,
Tee Company: 9.65x + 43
Shirtz Company: 8.40x + 58
If you were to buy one shirt from each company these would be the prices,
Tee Company: $52.65
Shirtz Company: $66.40
I would recommend the Tee Company since the price per shirt is cheaper.
The overtime rate is an added percentage to the regular rate. By how much percentage, this would depend on the place you're working. If your work in USA, under US federal labor and tax law, the rate would be 150%. Thus, the overtime rate would then be:
Overtime rate = $8.75/hour * 1.50 = $13.125/hour