While shopping for clothes, consuelo spend $3 less than twice what Brenda spent. Consuelo spent $81. How much did Brenda spend?

So,

Let Consuelo's spendings be c and Brenda's spendings be b.

We have our first equation: c = 2b - 3

And our second equation: c = 81

Substitute 81 for c

81 = 2b - 3

Add 3 to both sides

84 = 2b

Divide both sides by 2

42 = b

Brenda spent $42.