I don't now a lot about math even though I am the high In my class I want to now what is a=bxt+c

Generally when this equation is given the solution sought is for x.

So we begin by subtracting c from each side thus:

a - c = bxt + c - c = bxt + 0 = bxt

Restating:

a - c = bxt

Next we divide both sides by bt:

(a-c) / bt = x

We're done. Though to be completely rigorous, we might swap it around:

x = (a-c) / bt