Help me? Show work so I can understand please and thank you!

I think this is correct, Answer is 21

Step-by-step explanation:

Step 1: Write down formula:                                                

Step 2: Plug in b and c value in formula:            

Step 3: Solve the exponents:                            

Step 4: Subtract 400 from both sides:                    

Step 5: Square root both sides:                                

Step 6: 29+21:                    50

Step 7: 50-29:                     21

if x = 1, then the answer is 3.5

x = -5

step-by-step explanation: