ive been trying to answer these questions all night, 19 of them for homework and ive got 8 done. only 8 to go...

10. Solve the following systems of equations by substitution. Show your work.

a) y= 2x+10

y = 130

b) 2a + 3b = 35

b=10

c) 3x+5y = 12

x+3y=6

can someone explain to me how to do this, i dont understand it! i know how to check if their right afterwards, but especially for a.. i dont get what to 'plug in' and all that..