4dI/dt + 600 I = 120

There is no further info on E so E= constant= 120

dI/dt + 150 I = 30

You need a coefficient of 1 in front of dI/dt or the integrating factor

won't work

In factor is e^(150t)

I*e^(150t) = integral(30e^(150t))= 1/50 e^(150t) + C

I = .02 + C e^(-150t)

I(0) = 0 C = -.02

I = .02 -.02 e^(-150t)

The limiting value is .02 now you can finish