What is 19-5/2x=34 pleas help mwe

How do you find principle and simple interest

If f(x)=3x+1 and f^-1=x-1/3, then f^-1(7)=

Write a polynomial function f of least degree that has rational coefficients, a leading coefficient of 1, and the given zeros. 3, 4+2i, 1+(sqrt)7 the answer is supposed to be: f(x)=x(^5)-13x(^4)+60x(^3)-82x(^2)- 144x+360 what am i doing wrong?