A student does a standard addition experiment by pipetting 25 mL of unknown, adding ferrous ion spikes and reagents, into a 100 mL volumetric flask and making up to the mark. The equation of the line has a slope of 0.103 and an intercept of 0.135 when the unit of concentration was in ppm. What is the concentration of Fe(II) in the unknown?
You can read more about standard additions here.
https://en.wikipedia.org/wiki/Standard_addition
y = mx + b is straight line.
m is slope = 0.103
b is the intercept = 0.135 and that will be - if the x intercept.
Then solve for x = concentration in ppm. It is unclear in the problem if the unit of ppm concn is in the diluted or undiluted sample; i.e. you may need to correct the initial sample.That is you have diluted it from 25 mL to 100 mL so the final concentration may be 4x that (that's 100 mL/25 mL).