i graphed cut-off voltage verses frequency. when i figured out plank's constant so when i calculated the slope of the function i got 4.224x10-15. My question is how to convert V.s to J.s for the value of h hereS the question FINDING A VALUE OF PLANK'S CONSTANT FORM YOUR GRAPH. calculate the slope of the graph (in V.s) plank's constant is the slope of a graph of maximum kinetic evergy vs frequency so we need to convert cut-off coltage to maximum kinetic energy. we can use the equation. eV=change in kinetic energy because cut-off voltage just stops the most energetic electron. convert the value of your slope to J.s and compare your value to the accepted value of plank's constant.