Solve each proportion
5/t = t/45

T = 15 or -15 because you solve proportions by cross multiplying. if you write it out in the following way:

5       t
__=  __
t       45

Then, cross multiplying involves multiplying the numerator( number at the top) of one of the fractions with the denominator (number at the bottom) of the other, and setting the two results equal to each other. You get t * t = 5 * 45 and end up with t^2 = 225 or t = (15 or -15)