Let x denote the amount invested at 6.75%. That means that (30,000 - x) will be invested at 5%.
The allocation of funds between the two investments which would produce an aggregate return of exactly 6.5% would be described by the equation
0.065 = 0.0675x + 0.05(30,000 - x). Solving for x gives the amount invested at 6.75% (with the remainder invested at 5%) that would yield exactly 6.5%.
Technically, the question would be modeled as an inequality, since the company needs the portfolio to yield at least 6.5%...
0.065 <= 0.0675x + 0.05(30,000 - x) which would give you an answer of the form that x must be >= to some amount. But in this case, after solving the equality, it's common sense that this would represent the minimum that must be invested at 6.75%, because any shifting of funds from the 6.75% over to the 5% would decrease the overall yield below the required target.