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-x^2+2x+15≥0⇔-(x+3)(x-5)≥0⇔x∈<-3,5>
2x-4>0⇔x>2⇔x∈(2,∞)
<-3,2>
2. f(x+1)=(x+1)^2+3(x+1)=x^2+5x+4
f(x+3)=(x+3)^2+3(x+3)=x^2+9x+18
f(x+1)=f(x+3)
x^2+5x+4=x^2+9x+18
4x=-14
x=-7/2