Trevor earned $826 on an account bearing 2.6 percent interest for 7 years

how much money did he invest? (p)

This is a simple interest formula computation.

Given:
Interest = 826
Interest rate = 2.6%
term = 7 years.

The simple interest formula is:

Interest = Principal * Interest rate * term

To solve for the principal:
Principal = Interest / (Interest rate * term)
Principal = 826 / (2.6% * 7)
Principal = 826 / 18.2%
Principal = 4,538.46

To check: Solve for the interest

I = 4,538.46 * 2.6% * 7
I = 117.999 * 7
I = 825.993 rounded up to 826.