Let's take option A =>

Taxes = 70% of Groceries. We don't know how much Bill spent on groceries.

Not sufficient.

Let's take option B =>

Taxes + Groceries = 40 % of $6000

Taxes + Groceries = $2400

Not sufficient.

Taking them both.

Two equations. Two unknowns.

Sufficient.

C.

Bunuel wrote:

Last month Bill spent $6,000. How much did bill spend on taxes last month?

(1) The total amount Bill spent on taxes was 70% of the amount he spent on groceries.

(2) 40% of the total amount Bill spent last month was spent on taxes and groceries.