It looks like you have an equation involving exponents. Let's clarify and solve it step by step.

The equation you provided seems to be:
\[ 5^x + 3 = 5^{155} \]
and
\[ x + 3 = 5^{15} \]

Let's solve these equations one at a time.

### Equation 1: \( 5^x + 3 = 5^{155} \)

1. Subtract 3 from both sides to isolate the exponential term:
\[ 5^x = 5^{155} - 3 \]

Since \( 5^{155} \) is a very large number, subtracting 3 from it will still leave a number very close to \( 5^{155} \). However, this equation doesn't simplify nicely without more context or approximations.

### Equation 2: \( x + 3 = 5^{15} \)

1. Subtract 3 from both sides to solve for \( x \):
\[ x = 5^{15} - 3 \]

So, \( x \) is:
\[ x = 5^{15} - 3 \]

### Conclusion

For the first equation, \( 5^x + 3 = 5^{155} \), solving for \( x \) exactly is complex without approximations or numerical methods.

For the second equation, \( x + 3 = 5^{15} \), the solution is:
\[ x = 5^{15} - 3 \]

If you meant to combine these equations or if there was a typo, please clarify, and I can help further.