What is (x+3) (x-1)

Yes this is algebra.

When ever you use letters inplace of numbers it is algebra :)

(x+3) (x-1)

Take the first term from the first bracket  that is   x and times it by the second bracket.  x(x-1)

now take the second term from the first bracket that is +3 and times it by the second bracket.  +3(x-1)

so you have

$$\\ x(x-1) +3(x-1)\\ =x^2-1x+3x-3\\ =x^2+2x-3$$

Melody is this algebra?