using y mx b on a data for ph experiment

To help me figure out a way to solve these problems. I’m not sure what number goes where. I’ve done some of it and I’m not sure if I’m doing it right

Y=1.4431x+0.143

I need to solve for x

It says to substitute absorbance values for y to get x

Ex. Ph is 4.0
absorbance is .11

time is 900(Seconds)

X will be the Dopachrome concentration #
then says to divide concentrations by time to get the rate.