why do the questions you have use partial derivatives? since those aren't dealt with in HSC maths I'll just use the usual notation:
\frac{dx}{d t} = (t-3)^2\\ \int \frac{dx}{d t} dt = \int (t-3)^2dt\\ \int dx = \frac {1}{3}(t-3)^3 + C\\ x = \frac {1}{3}(t-3)^3 + C
when t = 0, x = 7
7 =...