To calculate V1 and R3 in a parallel circuit with R1 at 8 ohms and I1 at 0.75A, the voltage V1 is determined using Ohm's Law, yielding 6V. R2, with a value of 5 ohms, has a potential difference of 30V, leading to 24V across R3. The current through R3 is 6A, allowing for its calculation as 4 ohms. The total resistance in the parallel circuit is derived from the formula 1/Rt = 1/R2 + 1/R3. Ultimately, the calculated values for V1 and R3 are 6V and 1 ohm, respectively.