I will guess that "r1" is supposed to be "R1", "r2" is supposed to be "R2", and "i" is supposed to be "I". (This is not standard mathematical practice, is why I'm specifying.) I will guess that punctuation and capitalization were omitted. I will guess that the numbers after the letters are meant to be subscripted. (Superscripting is usual, is why I'm specifying.) I will guess that grouping symbols were omitted. (This will completely change the meaning of the equation.) In other words, I'm guessing that the exercise was meant to have been written as "Given I = v/(R1 + R2), I = 0.4, v = 100, and R1 = 200, solve the equation for the value of R2."
If this is incorrect, kindly please reply with corrections. Either way, please reply showing your efforts so far (starting with plugging the given values into the given equation):
. . . . .\(\displaystyle 0.4\, =\, \dfrac{100}{200\, +\, R_2}\)
...so we can see where you're having trouble. Thank you!