There's a cute "X-diagram" tool that can be used for solving mixture problems of all kinds. (See the attachment.) The numbers on the left represent the percentages of the constituents of the mix. The number in the center is the desired percentage in the resultant mix. The numbers on the right in the X are the differences along the diagonals. (6-5=1, 5-3=2)

These are also the proportions of the constituents in the final mix. That is, there are 2 parts of spice that is 6% salt for each 1 part of spice that is 3% salt. So, we need half as much of the latter as the former.

Half of 175 ounces is 87.5 ounces, the amount of 3% spice that is needed.

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If you really want to write an equation, you can let x represent the amount of 3% spice needed. Then the amount of salt in the mix is ...

... 175×6% +x×3% = (175+x)×5%

Dropping the percent symbols and eliminating parentheses, we have

... 1050 +3x = 875 +5x

... (1050 -875) = (5x -3x) . . . . . add -875-2x

... 175/2 = x = 87.5 . . . . . . . . . . divide by the coefficient of x