how to enable button after checked

1
2<input onclick="alert('thank you for agreeing to the terms and conditions')" type="submit" name="sendNewSms" class="inputButton" id="sendNewSms" value=" i agree to the terms and conditions " />
3
4<input type="checkbox"  onchange="document.getElementById('sendNewSms').disabled = !this.checked;" />
5<!--this will only work if u click the up arrow button on the right side--> 

1<input type="submit" name="sendNewSms" class="inputButton" id="sendNewSms" value=" Send " />
2
3<input type="checkbox"  onchange="document.getElementById('sendNewSms').disabled = !this.checked;" />