Cyclists were 3/5 finished with their ride when they reached the 15-kilometer mark. how long was their ride

The parabola opens: y=√x-4 (principal square root) up down right left

How do i do this? i don’t understand

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is