I don't understand how to do this physics problem HELP ME please

A loop circuit has a resistance of R1 and a
current of 2.1 A. The current is reduced to
1.4 A when an additional 1.2
resistor is
added in series with R1.
What is the value of R1? Assume the inter-
nal resistance of the source of emf is zero.
Answer in units of ohms

V=IR
V = R1 * 2.1
V = (R1 + 1.2) * 1.4 (extra resistor)
putting voltage the same in both cases
R1 * 2.1 = (R1 + 1.2) * 1.4 = 1.4 * R1 + 1.68
R1 * (2.1 - 1.4) = 1.68
R1 = 2.4

so V = 5.04