1/27+9^4x+5

And what is the question or problem?

what is x3-7y=21

It looks like you have provided two separate expressions in your message. Let's break down each one and find the solution to the equations:

1) 1/27 + 9^4x + 5:
To solve this expression, we need to simplify it by following the order of operations (PEMDAS/BODMAS). First, we'll calculate the exponent, then perform the addition and division.

The equation is:
1/27 + 9^(4x) + 5

To simplify 9^(4x), we need to evaluate the exponential term first:
9^(4x) = (3^2)^(4x) = 3^(2 * 4x) = 3^(8x)

Now, we have:
1/27 + 3^(8x) + 5

To add fractions like 1/27 and 5, we need a common denominator:
1/27 + 5(27/27) = 1/27 + 135/27 = (1 + 135)/27 = 136/27

Therefore, the simplified expression is:
136/27 + 3^(8x)

2) x^3 - 7y = 21:
This equation appears to be a polynomial equation in two variables, x and y. However, you haven't mentioned any specific instructions or constraints.

To solve for one variable in terms of the other or find numerical solutions, we need additional information or another equation. Without further details or constraints, it is not possible to determine the exact values of x and y.

If you can provide more information or any additional equations, I would be happy to assist you further.