Petra started with a number added 3 multiplied the result by 4 subtracted 6 and multiplied that result by 3 her final answer was 90 what did she begin with?

We don't know the number she started with, so let's call it x.

Petra started with a number added 3: x + 3

multiplied the result by 4: 4 (x + 3)

subtracted 6: 4 (x + 3) - 6

and multiplied that result by 3: 3[4 (x + 3) - 6]

her final answer was 90: 3[4 (x + 3) - 6] = 90

Now we solve the equation for x.

3[4 (x + 3) - 6] = 90

4 (x + 3) - 6 = 30

4 (x + 3) = 36

x + 3 = 9

x = 6

The number is 6.