Answer:
$7
Explanation:
Robert = $10 or 2x-4
Eric = x (variable), 2x-4=10 (twice minus 4)
Now just manage the equation
2x-4=10 (substitute by adding 4 to both sides)
2x = 4 + 10
2x =14 (substitute by diving by 2 getting rid of x2)
x = 14/2
x = 7
Eric earnt 7 dollars.