[Math] Arithmetic Progression. Find 1st term and common difference

Question

The seventh term of an A.P. is 15 and the sum of the first seven terms is 42. Find the first term and common difference.

How do I find the first term and common difference with only 1 term given?

Answer 1

Note: use that $$a_7=a_1+6d=15$$ and $$7a_1+21d=42$$