51 is 75% of what number

Given the function f(x) = 3x + 1, evaluate f(a + 1). a. 3a + 1 b. a + 2 c. 3a + 4

Ihonestly have no clue if it's each or none.

Given that sin∅ = 1/4, 0 < ∅ < π/2, what is the exact value of cos∅? a. (√4)/4 b. (√15)/4 c. (4π)/2 d. (4√2)/4