Question

Please help me with this

Answer 1

since ABC is equal to DCB, you can use the angles given in triangle ABC in triangle DCB. Since the sum of the angles in a triangle is 180, the equation would be this:
90+46+x=180 

Solve for x in that and you're good :)

Answer 2

use trig so the side with 4x-7 is also equal to 21sin(46)=15 so 4x-7=15.1. thus x=5.52 which is around 6