This law is known as Ohm's Law

where V is the voltage, I is the current, and R is the resistance
Science lesson aside, this is easy. In this case, voltage is constant.
First, if we multiply 5 A by 50 ohms, we get 250 V. That is our goal.
What can be multiplied by 2 to get to 250? 125. So the resistance is 125 ohms, given that the voltage is constant, and the current is 2 amperes