Isolate the Power First

In 5 × 3^x = 40 the power is not alone: a 5 multiplies it. A log undoes a bare power, so first get 3^x by itself, exactly as you would get x by itself. Divide both sides by 5: 3^x = 8.

Now take log base 3 of both sides: x = log_3(8), about 1.89. If a number is added as well, as in 4 × 2^x + 3 = 15, undo the adding first, then the multiplying.

One trap: 5 × 3^x is not 15^x. The exponent belongs to the 3 alone, because powers come before multiplying, so the 5 and the 3 cannot be combined first.