P = I^2 R
Are we to assume that P, I, and R are all functions of a common variable, maybe 't'?
If so, we have by the Product Rule dP = I^2 dR + 2*I*dI*R
I has 0.75%, or dI/I = 0.0075 ==> I = dI/*0.0075
R has 0.25%, or dR/R = 0.0025 ==> R = dR/0.0025
Putting it all together
dP = I^2 dR + 2*I*dI*R = (dI/*0.0075)^2 dR + 2*(dI/*0.0075)*dI*dR/0.0025 = [dR * dI^2 / 0.0075]*[1/0.0075 + 2/0.0025]
For Relative Error, calculate dP/P and see all the stuff that cancels out.