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In this kind of simplification, you always simplify out by using the common factor in the expression. In this case for 4x+3, there is no common factor that can be used to divide both 4x and 3. It would have been possible if the term was 4x+4 as 4 would have been a common factor and thus giving 4(x+1) but for the case of 4x+3, that answer is simplified and nothing more can be done about it.

5+6y=9y+2+3(1-y)

opening the brackets

5+6y=9y+2+3-3y

putting like terms together

6y-9y+3y=2+3-5

using BODMAS

9y-9y=0

2+3+5=0

thus the answer reduces to zero

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