Assuming that the artist wants to paint 6 sides of the cube, the formula to count the surface area of the cube would be: 6* edge length^2. If the edge length is 10 feet, then the area would be:
area= 6*edge length^2
area= 6* 10^2 feet
area= 6*100feet^2= 600 feet^2
If one gallon of paint can be used for 350 feet^2, the amount of paint needed would be: 600feet^2/ (350ft^2/ gallon)= 1.71 gallon