You can put this solution on YOUR website! All we have to do is plug in m+1 for every x in the problem.
f(x) = x^2+3x
f(m+1) = (m+1)^2+3(m+1) foiling and distributing we get
= m^2+2m+1+3m+3 Simplifying you get
= m^2+5m+4
and where done, unless your teacher wants it in factored form then its (m+1)(m+4).