Final answer:
In the equation y=21x, y varies directly with x since y is equal to 21 times the value of x, indicating a constant and proportional relationship between the two variables.
Step-by-step explanation:
To determine whether x and y vary directly, inversely, or jointly in the equation y=21x, let's analyze the form of the equation. In the equation given, y is equal to 21 times x. This type of equation indicates a direct variation between x and y because as x increases, y increases proportionally, and vice versa. There is a constant ratio between y and x, which in this case, is 21. This means that for any value of x, y is always 21 times x. There is no sign of an inverse or joint variation in the equation provided.