Why is y=3x a linear function

Larger pets need to be transported in larger portable kennels. using a graphing calculator and the relationship between the width, length, and height, find the dimensions of a portable kennel whose volume is 7.4 ft3. the dimensions of a portable kennel can be expressed as width x, length x + 0.4, and height x − 0.2? what are the dimensions of a portable kennel with a volume of 7.4 ft3? use a graphing calculator to solve. round the dimensions to the nearest tenth.