Answer:
B
Step-by-step explanation:
Given
7 | 3f + 4 | = 91 ← divide both sides by 7
| 3f + 4 | = 13
The absolute value function always returns a positive value, but the expression inside can be positive or negative. Thus there are 2 possible solutions.
Solve 3f + 4 = 13 → 3f = 13 - 4 = 9 ⇒ f = 3
Solve 3f + 4 = - 13 ⇒ 3f = - 13 - 4 = - 17 ⇒ f = -
As a check
Substitute these values into the left side of the equation and if equal to the right side then they are the solutions.
f = 3 : 7| 9 + 4| = 7| 13| = 7 × 13 = 91 ← True
f = -
7 | - 17 + 4| = 7| - 13| = 7 × 13 = 91 ← True
Hence the correct solutions are B