Since c represents the number of pounds of chocolates
Since n represents the number of pounds of nuts
Since Clint is making 10 pounds of them, then

Since the cost of 1 pound of chocolates is $3.00
Since the cost of 1 pound of nuts is $6.00
Since the Clint budget is $5.1 per pound, then

Multiply c by 3 and n by 6, then add the products and equate the sum by 51

The system of equations is
c + n = 10
3c + 6n = 51
Let us solve them
Multiply equation (1) by -3 to make the coefficient of c equal in values and different in signs

Add equations (2) and (3)

Divide both sides by 3 to find n

Substitute n by 7 in equation (1)

Subtract 7 from both sides

He should use 3 pounds of chocolate
He should use 7 pounds of nuts
If his budget is $51