Let x = amount of sales (in dollars)
The salary is $400 and there's an additional 0.06x dollars added on to get to the goal of 790. The equation is therefore
400+0.06x = 790
Let's solve for x
400+0.06x = 790
400+0.06x-400 = 790-400
0.06x = 390
0.06x/0.06 = 390/0.06
x = 6500
The final answer is 6500
This means he must have $6,500 in sales.