Solve the equation for y.

9x+y=10

Y=-9x+10 is the equation solved for y. 

You said              9x + y  = 10

Subtract  9x
from each side:           y  =  -9x + 10  

$250 deductible means jane must pay the first $250 of her bill. so the insurance company paid = (80% of bill) - deductible = (0.8 x $2550) - $250 = $2040 - $250 = $1790 and jane's cost is = (bill) - (insurance payment) = $2550 - $1790 = $760