Answer:
10x + 50 = 90; 4 hours
Explanation:
Let
x -----> the number of hours
y ----> the total charge
we know that
The total charge is equal to the number of hours multiplied by the rate of $10 per hour plus a flat fee of $50
so
The linear equation that represent this problem is

For y=$90
substitute in the equation and solve for x

Subtract 50 both sides


Divide by 10 both sides

therefore
10x + 50 = 90; 4 hours