HELP ASAP PLEASE

Question

Please help and view the question above - what is the mistake? what is the real answer?

If cosx = sqrt(7)/4 and 3pi/2<x<2pi find tantan(x/2)

Ms. Jone's work:

Line 1: sqrt(7)^2+x^2=4^2, x=3

Line 2: sinx = 3/4

Line 3: tan(x/2) = (1-(sqrt(7)/4)/3/4

Line 4: tan(x/2) = (4-sqrt(7)/4*4/3

Line 5: Answer: tan(x/2)=4+sqrt(7)/3

Answer 1

Line 3 is not the correct formula for tan (x/2).