To solve this function, all we need to do is substitute -4.3 in for wherever "x" appears and then simplify.
f(x) = 7.45x + 33.7; x = -4.3
f(-4.3) = 7.45(-4.3) + 33.7
f(-4.3) = -32.035 + 33.7
f(-4.3) = 1.665
This means that, when x = -4.3, f(x) = 1.665.
Note: It may help to remember that, in a function such as this, f(x) can be thought of as the "y" value that corresponds to the given "x" value (in fact, when writing it out with words, f(x) is the same as "function of x"). And, if you are given a value for f(x), then the opposite is true. Just substitute in the value given for f(x) and solve for x.
Hope this helps!