book Maths Puzzle Numerical Ability

CAN YOU PLEASE GIVE ME THE ANSWER FOR 7a to the third power-2a squared+6-7a squared+2a-4

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book Other Question

A two digit number is seven times the sum of it's digits. The tens digit is 3 more then the units digit. What is the number? One person can do a job in 2 hours, Another can do the job in 3 hours, how long will it take if they do the job together.

What is the formula to solve this type of question?
My attempts seem futile.