A potential divider contains three resistors of 4 Ω, 7 Ω and
9 Ω. If the voltage across the 7 Ω is 21 V, what is the supply voltage?

Please help me :(

Large electric fields in cell membranes cause ions to move through the cell wall. the field strength in a typical membrane is 1.0 x 10 7 n/c. what is the magnitude of the force on a calcium ion with charge +e? what is its acceleration? is it possible for a particle with the same charge as as calcium to have a different acceleration if placed at this location? what would change the acceleration: mass of the particle? diameter of the particle?