Please make sure you are using the latest version of MathWallet
1 Get BTC address from Coinbase
Login your Coinbase account
Click the 'Receive' button in BTC Account
Copy the BTC address and send to your mobile phone
2 Send BTC from your MathWallet to this address
Now, open the MathWallet app and follow steps below
Paste the Coinbase BTC address here, enter the amount, leave the Fee as default.
Click Next and enter the wallet password.
Note: Please test with a small amount first.