It is given that \(\mathrm f(x)=\ln(2x+5)\) for \(x>a\), where \(a\) is a constant.
Write down the least possible value of \(a\).[1]
Using your value of \(a\), write down the range of \(\mathrm f\).[1]
It is also given that \(\mathrm g(x)=x^2+1\) for \(x\in\mathbb R\).
Using your value of \(a\), solve the equation \(\mathrm{fg}(x)=4\).
Give your answers in exact form.[3]