f(x) is followed by a description of the function in terms of a parameter or variable called x.
f(-1) tells us to use the description of f and plug into the parameter x the number -1. So wherever there's an x replace it with -1.
Here f(x) is defined as the polynomial 6x2-5x+6, so if we replace x by -1 we get:
6(-1)2-5(-1)+6=6+5+6=17. (Notice that when two negatives are multiplied the result is positive, so (-1)2=-1×(-1)=1 and -5×(-1)=5.)
Therefore, f(-1)=17.