'Mayest' in the Bible

Only be strong and very courageous, to observe to do according to all the law, which Moses my servant commanded thee: turn not from it to the right hand or to the left, that thou mayest have good success whithersoever thou goest.

This book of the law shall not depart out of thy mouth, but thou shalt meditate thereon day and night, that thou mayest observe to do according to all that is written therein: for then thou shalt make thy way prosperous, and then thou shalt have good success.