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.
